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#### VizieR Online Data Catalog: AzTEC 1.1mm observations of 17 radio sources (Zeballos+, 2018)

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Zeballos, M. ; Aretxaga, I. ; Hughes, D. H. ; Humphrey, A. ; Wilson, G. W. ; Austermann, J. ; Dunlop, J. S. ; Ezawa, H. ; Ferrusca, D. ; Hatsukade, B. ; Ivison, R. J. ; Kawabe, R. ; Kim, S. ; Kodama, T. ; Kohno, K. ; Montana, A. ; Nakanishi, K. ; Plionis, M. ; Sanchez-Arguelles, D. ; Stevens, J. A. ; Tamura, Y. ; Velazquez, M. ; Yun, M. S.
VizieR On-line Data Catalog: J/MNRAS/479/4577. Originally published in: 2018MNRAS.479.4577Z
Publication year: 2022

We targeted fields centred on luminous AGN at 0.5<z<6.3 which were known or suspected to be hosted by massive galaxies, as potential signposts of high-redshift overdensities. The ACES protocluster sample was surveyed at a wavelength of 1.1mm with the AzTEC camera as a visiting instrument at the 15-m James Clerk Maxwell Telescope (JCMT;FWHM=18arcsec) in Hawaii and at the 10-m Atacama Submillimeter Telescope Experiment (ASTE; FWHM=30arcsec) in the northern part of Chile. The field towards 4C+41.17 was observed at the JCMT in 2005 December, and the other 16 fields were observed at the ASTE from 2007 May to October and 2008 July to December. Table 3 shows the source catalogue for the PKS1138-262 field. This list is in decreasing order of S/N and includes the measured 1.1mm flux densities and their deboosted fluxes. Tables A1 to A16 show the catalogues for the other ACES protoclusters.

#### Using our newest VLT-KMOS HII galaxies and other cosmic tracers to test the Lambda cold dark matter tension

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Mehrabi, Ahmad ; Basilakos, Spyros search by orcid ; Tsiapi, Pavlina ; Plionis, Manolis ; Terlevich, Roberto search by orcid ; Terlevich, Elena ; Gonzalez Moran, Ana Luisa ; Chavez, Ricardo search by orcid ; Bresolin, Fabio ; Fernandez Arenas, David ; Telles, Eduardo
Monthly Notices of the Royal Astronomical Society, Volume 509, Issue 1, pp.224-231
Publication year: 2022

We place novel constraints on the cosmokinetic parameters by using a joint analysis of the newest Very Large Telescope (VLT)-K-band Multi Object Spectrograph (KMOS) HII galaxies (HIIG) with the Type Ia Supernovae (SNIa) Pantheon sample. We combine the latter data sets in order to reconstruct, in a model-independent way, the Hubble diagram to as high redshifts as possible. Using a Gaussian process, we derive the basic cosmokinetic parameters and compare them with those of Lambda cold dark matter (ΛCDM). In the case of SNIa, we find that the extracted values of the cosmokinetic parameters are in agreement with the predictions of ΛCDM model. Combining SNIa with high-redshift tracers of the Hubble relation, namely HIIG data, we obtain consistent results with those based on ΛCDM as far as the present values of the cosmokinetic parameters are concerned, but find significant deviations in the evolution of the cosmokinetic parameters with respect to the expectations of the concordance ΛCDM model.

#### Host galaxy properties of X-ray active galactic nuclei in the local Universe

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Koutoulidis, L. ; Mountrichas, G. ; Georgantopoulos, I. ; Pouliasis, E. ; Plionis, M.
Astronomy & Astrophysics, Volume 658, id.A35, 7 pp.
Publication year: 2022

We study the host galaxy properties of active galactic nuclei (AGN) that have been detected in X-rays in the nearby Universe (z < 0.2). For that purpose, we use the catalogue provided by the ROSAT-2RXS in the 0.1-2.4 keV energy band, one of the largest X-ray datasets with spectroscopic observations. Our sample consists of ∼900 X-ray AGN. The catalogue provides classification of the sources into type 1 and 2 based on optical spectra. Approximately 25% of the AGN are type 2. We use the available optical, near-IR, and mid-IR photometry to construct spectral energy distributions (SEDs). We measure the stellar mass (M*) and star formation rate (SFR) of the AGN by fitting these SEDs with the X-CIGALE code. We compare the M* and SFR of the two AGN populations, taking their different redshift and luminosity distributions into account. Based on our results, type 2 AGN tend to live in more massive galaxies compared to their type 1 counterparts (log [M(M)] = 10.49−0.10+0.16 vs. 10.23−0.08+0.05), in agreement with previous studies at higher redshifts. In terms of SFRs, our analysis shows that, in the nearby Universe, the number of X-ray AGN that live in quiescent systems is higher compared to at higher redshifts, in accordance with previous studies in the local Universe. However, the majority of AGN (∼75%) live inside or above the main sequence.

#### Understanding X-ray and optical selection of galaxy clusters: a comparison of the XXL and CAMIRA cluster catalogues obtained in the common XXL-HSC SSP area

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Willis, J. P. search by orcid ; Oguri, M. ; Ramos-Ceja, M. E. search by orcid ; Gastaldello, F. search by orcid ; Sereno, M. search by orcid ; Adami, C. ; Alis, S. search by orcid ; Altieri, B. ; Chiappetti, L. search by orcid ; Corasaniti, P. S. ; Eckert, D. search by orcid ; Ettori, S. search by orcid ; Garrel, C. search by orcid ; Giles, P. ; Lefevre, J. ; Faccioli, L. ; Fotopoulou, S. search by orcid ; Hamabata, A. ; Koulouridis, E. ; Lieu, M. ; Lin, Y. -T. search by orcid ; Maughan, B. search by orcid ; Nishizawa, A. J. search by orcid ; Okabe, T. ; Okabe, N. search by orcid ; Pacaud, F. search by orcid ; Paltani, S. ; Pierre, M. ; Plionis, M. ; Poggianti, B. ; Pompei, E. ; Sadibekova, T. ; Umetsu, K. search by orcid ; Valageas, P.
Monthly Notices of the Royal Astronomical Society, Volume 503, Issue 4, pp.5624-5637
Publication year: 2021

Large samples of galaxy clusters provide knowledge of both astrophysics in the most massive virialized environments and the properties of the cosmological model that defines our Universe. However, an important issue that affects the interpretation of galaxy cluster samples is the role played by the selection waveband and the potential for this to introduce a bias in the physical properties of clusters thus selected. We aim to investigate waveband-dependent selection effects in the identification of galaxy clusters by comparing the X-ray MultiMirror (XMM) Ultimate Extra-galactic Survey (XXL) and Subaru Hyper Suprime-Cam (HSC) CAMIRA cluster samples identified from a common 22.6 deg2 sky area. We compare 150 XXL and 270 CAMIRA clusters in a common parameter space defined by X-ray aperture brightness and optical richness. We find that 71/150 XXL clusters are matched to the location of a CAMIRA cluster, the majority of which (67/71) display richness values N > 15 that exceed the CAMIRA catalogue richness threshold. We find that 67/270 CAMIRA clusters are matched to the location of an XXL cluster (defined within XXL as an extended X-ray source). Of the unmatched CAMIRA clusters, the majority display low X-ray fluxes consistent with the lack of an XXL counterpart. However, a significant fraction (64/107) CAMIRA clusters that display high X-ray fluxes are not associated with an extended source in the XXL catalogue. We demonstrate that this disparity arises from a variety of effects including the morphological criteria employed to identify X-ray clusters and the properties of the XMM PSF.

#### The XXL survey: XLVI. Forward cosmological analysis of the C1 cluster sample

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Garrel, Christian ; Pierre, Marguerite ; Valageas, Patrick ; Eckert, Dominique search by orcid ; Marulli, Federico ; Veropalumbo, Alfonso ; Pacaud, Florian ; Clerc, Nicolas ; Sereno, Mauro ; Umetsu, Keiichi ; Moscardini, Lauro ; Bhargava, Sunayana ; Adami, Christophe ; Chiappetti, Lucio search by orcid ; Gastaldello, Fabio ; Koulouridis, Elias ; Le Fevre, Jean-Paul ; Plionis, Manolis
eprint arXiv:2109.13171
Publication year: 2021

We present the forward cosmological analysis of an XMMXMM selected sample of galaxy clusters out to a redshift of unity. Following our previous 2018 study based on the dn/dz quantity alone, we perform an upgraded cosmological analysis of the same XXL C1 cluster catalogue (178 objects), with a detailed account of the systematic errors. We follow the ASpiX methodology: the distribution of the observed X-ray properties of the cluster population is analysed in a 3D observable space (count rate, hardness ratio, redshift) and modelled as a function of cosmology. Compared to more traditional methods, ASpiX allows the inclusion of clusters down to a few tens of photons. We obtain an improvement by a factor of 2 compared to the previous analysis by letting the normalisation of the M-T relation and the evolution of the L-T relation free. Adding constraints from the XXL cluster 2-point correlation function and the BAO from various surveys decreases the uncertainties by 23 and 53 % respectively, and 62% when adding both. Switching to the scaling relations from the Subaru analysis, and letting free more parameters, our final constraints are σ8σ8 = 0.99+0.140.230.99−0.23+0.14ΩmΩm = 0.296 ±± 0.034 (S8=0.98+0.110.21S8=0.98−0.21+0.11) for the XXL sample alone. Finally, we combine XXL ASpiX, the XXL cluster 2-point correlation function and the BAO, with 11 free parameters, allowing for the cosmological dependence of the scaling relations in the fit. We find σ8σ8 = 0.793+0.0630.120.793−0.12+0.063ΩmΩm = 0.364 ±± 0.015 (S8=0.872+0.0680.12S8=0.872−0.12+0.068), but still compatible with Planck CMB at 2.2σσ. The results obtained by the ASpiX method are promising; further improvement is expected from the final XXL cosmological analysis involving a cluster sample twice as large. Such a study paves the way for the analysis of the eROSITA and future Athena surveys.

#### The physical properties of local (U)LIRGs: A comparison with nearby early- and late-type galaxies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Paspaliaris, E. -D. search by orcid ; Xilouris, E. M. search by orcid ; Nersesian, A. search by orcid ; Masoura, V. A. search by orcid ; Plionis, M. search by orcid ; Georgantopoulos, I. ; Bianchi, S. search by orcid ; Katsioli, S. search by orcid ; Mountrichas, G.
Astronomy & Astrophysics, Volume 649, id.A137, 33 pp.
Publication year: 2021

Aims: In order to pinpoint the place of the (ultra-) luminous infrared galaxies ((U)LIRGs) in the local Universe, we examine the properties of a sample of 67 such nearby systems and compare them with those of 268 early- and 542 late-type, well studied, galaxies from the DustPedia database.
Methods: We made use of multi-wavelength photometric data (from the ultra-violet to the sub-millimetre), culled from the literature, and the CIGALE spectral energy distribution fitting code to extract the physical parameters of each system. The median spectral energy distributions as well as the values of the derived parameters were compared to those of the local early- and late-type galaxies. In addition to that, (U)LIRGs were divided into seven classes, according to the merging stage of each system, and variations in the derived parameters were investigated.
Results: (U)LIRGs occupy the high-end’ on the dust mass, stellar mass, and star-formation rate (SFR) plane in the local Universe with median values of 5.2 × 107 M, 6.3 × 1010 M, and 52 M yr−1, respectively. The median value of the dust temperature in (U)LIRGs is 32 K, which is higher compared to both the early-type (28 K) and the late-type (22 K) galaxies. The dust emission in PDR regions in (U)LIRGs is 11.7% of the total dust luminosity, which is significantly higher than early-type (1.6%) and late-type (5.2%) galaxies. Small differences in the derived parameters are seen for the seven merging classes of our sample of (U)LIRGs with the most evident one being on the SFR, where in systems in late merging stages (M3′ and M4′) the median SFR reaches up to 99 M yr−1 compared to 26 M yr−1 for the isolated ones. In contrast to the local early- and late-type galaxies where the old stars are the dominant source of the stellar emission, the young stars in (U)LIRGs contribute with 64% of their luminosity to the total stellar luminosity. The fraction of the stellar luminosity absorbed by the dust is extremely high in (U)LIRGs (78%) compared to 7% and 25% in early- and late-type galaxies, respectively. The fraction of the stellar luminosity used to heat up the dust grains is very high in (U)LIRGs, for both stellar components (92% and 56% for the young and the old stellar populations, respectively) while 74% of the dust emission comes from the young stars.

#### Independent cosmological constraints from high-z H II galaxies: new results from VLT-KMOS data

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
González-Morán, Ana Luisa ; Chávez, Ricardo search by orcid ; Terlevich, Elena ; Terlevich, Roberto ; Fernández-Arenas, David search by orcid ; Bresolin, Fabio search by orcid ; Plionis, Manolis ; Melnick, Jorge ; Basilakos, Spyros search by orcid ; Telles, Eduardo
Monthly Notices of the Royal Astronomical Society, Volume 505, Issue 1, pp.1441-1457
Publication year: 2021

We present independent determinations of cosmological parameters using the distance estimator based on the established correlation between the Balmer line luminosity, L(H β), and the velocity dispersion (σ) for H II galaxies (HIIG). These results are based on new VLT-KMOS high spectral resolution observations of 41 high-z (1.3 ≤ z ≤2.6) HIIG combined with published data for 45 high-z and 107 z ≤0.15 HIIG, while the cosmological analysis is based on the MultiNest Markov Chain Monte Carlo (MCMC) procedure not considering systematic uncertainties. Using only HIIG to constrain the matter density parameter (Ωm), we find Ωm=0.244+0.0400.049Ωm=0.244−0.049+0.040 (stat), an improvement over our best previous cosmological parameter constraints, as indicated by a 37 per cent increase of the figure of merit. The marginalized best-fitting parameter values for the plane {Ωm; w0} = {0.249+0.110.065;1.18+0.450.41}{0.249−0.065+0.11;−1.18−0.41+0.45} (stat) show an improvement of the cosmological parameters constraints by 40 per cent. Combining the HIIG Hubble diagram, the cosmic microwave background (CMB) and the baryon acoustic oscillation (BAO) probes yields Ωm = 0.298 ± 0.012 and w0 = -1.005 ± 0.051, which are certainly compatible – although less constraining – than the solution based on the joint analysis of Ia supernovae (SNIa), CMB and BAO measurements. An attempt to constrain the evolution of the dark energy with time (CPL model), using a joint analysis of the HIIG, CMB, and BAO measurements, shows a degenerate 1σ contour of the parameters in the {w0, wa} plane.

#### Host galaxy and orientation differences between different AGN types

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Gkini, Anamaria ; Plionis, Manolis search by orcid ; Chira, Maria ; Koulouridis, Elias
Astronomy & Astrophysics, Volume 650, id.A75, 12 pp.
Publication year: 2021

Aims: The main purpose of this study is to investigate aspects regarding the validity of the active galactic nucleus (AGN) unification paradigm (UP). In particular, we focus on the AGN host galaxies, which according to the UP should show no systematic differences depending on the AGN classification.
Methods: For the purpose of this study, we used (a) the spectroscopic Sloan Digital Sky Survey (SDSS) Data Release 14 catalogue, in order to select and classify AGNs using emission line diagnostics, up to a redshift of z = 0.2, and (b) the Galaxy Zoo Project catalogue, which classifies SDSS galaxies in two broad Hubble types: spirals and ellipticals.
Results: We find that the fraction of type 1 Seyfert nuclei (Sy1) hosted in elliptical galaxies is significantly larger than the corresponding fraction of any other AGN type, while there is a gradient of increasing spiral-hosts from Sy1 to LINER, type 2 Seyferts (Sy2) and composite nuclei. These findings cannot be interpreted within the simple unified model, but possibly by a co-evolution scheme for supermassive black holes and galactic bulges. Furthermore, for the case of spiral host galaxies we find the Sy1 population to be strongly skewed towards face-on configurations, while the corresponding Sy2 population range in all host galaxy orientation configurations has a similar, but not identical, orientation distribution to star-forming galaxies. These results also cannot be interpreted by the standard unification paradigm, but point towards a significant contribution of the galactic disc to the obscuration of the nuclear region. This is also consistent with the observed preference of Sy1 nuclei to be hosted by ellipticals, that is, the dusty disc of spiral hosts contributes to the obscuration of the broad-line region, and thus relatively more ellipticals are expected to appear hosting Sy1 nuclei.

#### GAMA/XXL: X-ray point sources in low-luminosity galaxies in the GAMA G02/XXL-N field N

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Nwaokoro, E.; Phillipps, S.; Young, A. J.; Baldry, I.; Bongiorno, A.; Bremer, M. N.; Brown, M. J. I.; Chiappetti, L.; De Propris, R.; Driver, S. P.; Elyiv, A.; Fotopoulou, S.; Giles, P. A.; Hopkins, A. M.; Maughan, B.; McGee, S.; Pacaud, F.; Pierre, M.; Plionis, M.; Poggianti, B. M.; Vignali, C.
Monthly Notices of the Royal Astronomical Society, Vol. 502, p.3101-3112
Publication year: 2021

Relatively few X-ray sources are known that have low-mass galaxies as hosts. This is an important restriction on studies of active galactic nuclei (AGNs), hence black holes, and of X-ray binaries (XRBs) in low-mass galaxies; addressing it requires very large samples of both galaxies and X-ray sources. Here, we have matched the X-ray point sources found in the XXL-N field of the XXL survey (with an X-ray flux limit of ∼6 × 10-15 erg s-1 cm-2 in the [0.5-2] keV band) to galaxies with redshifts from the Galaxy And Mass Assembly (GAMA) G02 survey field (down to a magnitude limit r = 19.8) in order to search for AGNs and XRBs in GAMA galaxies, particularly those of low optical luminosity or stellar mass (fainter than Mr = -19 or M109.5M ). Out of a total of 1200 low-mass galaxies in the overlap region, we find a total of 28 potential X-ray source hosts, though this includes possible background contaminants. From a combination of photometry (optical and infrared colours), positional information, and optical spectra, we deduce that most of the ≃20 X-ray sources genuinely in low-mass galaxies are high-mass X-ray binaries in star-forming galaxies. None of the matched sources in a low-mass galaxy has a BPT classification as an AGN, and even ignoring this requirement, none passes both criteria of close match between the X-ray source position and optical galaxy centre (separation ≤3 arcsec) and high [O III] line luminosity (above 1040.3 erg s-1).

#### Cosmological constraints using the newest VLT-KMOS H II galaxies and the full Planck CMB spectrum

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Tsiapi, Pavlina search by orcid ; Basilakos, Spyros search by orcid ; Plionis, Manolis ; Terlevich, Roberto search by orcid ; Terlevich, Elena ; Gonzalez Moran, Ana Luisa ; Chavez, Ricardo search by orcid ; Bresolin, Fabio search by orcid ; Fernandez Arenas, David search by orcid ; Telles, Eduardo
Monthly Notices of the Royal Astronomical Society, Volume 506, Issue 4, pp.5039-5045
Publication year: 2021

We present novel cosmological constraints based on a joint analysis of our H II galaxies (HIIG) Hubble relation with the full Planck cosmic microwave background (CMB) anisotropy spectrum and the baryon acoustic oscillations (BAO) probes. The HIIG span a large redshift range (0.088 ≤ z ≤ 2.5), reaching significantly higher redshifts than available Type Ia supernovae (SNeIa) and hence they probe the cosmic expansion at earlier times. Our independent constraints compare well with those based on the ‘Pantheon’ compilation of SNeIa data, which we also analyse. We find our results to be in agreement with the conformal Λ cold dark matter (ΛCDM) model within 1σ. We also use our HIIG data to examine the behaviour of the dark energy equation-of-state parameter under the Chevallier-Polarski-Linder (CPL) parametrization, w = w0 + waz/(1 + z), and find consistent results with those based on SNeIa, although the degeneracy in the parameter space and the individual parameter uncertainties, when marginalizing one over the other, are quite large.

#### The XXL Survey. XLIII. The quasar radio loudness dichotomy exposed via radio luminosity functions obtained by combining results from COSMOS and XXL-S X-ray selected quasars

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Ceraj, L.; Smolčić, V.; Delvecchio, I.; Butler, A.; Tisanić, K.; Delhaize, J.; Horellou, C.; Kartaltepe, J.; Kolokythas, K.; Leslie, S.; Marchesi, S.; Novak, M.; Pierre, M.; Plionis, M.; Vardoulaki, E.; Zamorani, G.
Astronomy & Astrophysics, Vol. 642, p.14
Publication year: 2020

#### Internal kinematics of giant H II regions in M101 with the Keck Cosmic Web Imager

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Fabio Bresolin, Luca Rizzi, I-Ting Ho, Roberto Terlevich, Elena Terlevich, Eduardo Telles, Ricardo Chávez, Spyros Basilakos, Manolis Plionis
Monthly Notices of the Royal Astronomical Society, Volume 495, Issue 4, July 2020, Pages 4347–4365
Publication year: 2020

## Summary

We study the kinematics of the giant H II regions NGC 5455 and NGC 5471 located in the galaxy M101, using integral field observations that include the H β and [O III]λλ 5007 emission lines, obtained with the Keck Cosmic Web Imager. We analyse the line profiles using both single and multiple Gaussian curves, gathering evidence for the presence of several expanding shells and moving filaments. The line decomposition shows that a broad (σ ≃ 30–50 km s−1) underlying component is ubiquitous, extending across hundreds of pc, while a large fraction of the narrow components have subsonic line widths. The supersonic turbulence inferred from the global line profiles is consistent with the velocity dispersion of the individual narrow components, i.e. the global profiles likely arise from the combined contribution of discrete gas clouds. We confirm the presence of very extended (400–1200 km s−1) low-intensity line components in three bright star-forming cores in NGC 5471, possibly representing kinematic signatures of supernova remnants. For one of these, the known supernova remnant host NGC 5471 B, we find a significantly reduced [O III]/H β line ratio relative to the surrounding photoionized gas, due to the presence of a radiative shock at low metallicity. We explore the systematic width discrepancy between H I and [O III] lines, present in both global and individual spaxel spectra. We argue that the resolution of this long-standing problem lies in the physics of the line-emitting gas rather than in the smearing effects induced by the different thermal widths.

#### Independent cosmological constraints from high-z H II galaxies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Ana Luisa González-Morán, Ricardo Chávez, Roberto Terlevich, Elena Terlevich, Fabio Bresolin, David Fernández-Arenas, Manolis Plionis, Spyros Basilakos, Jorge Melnick, Eduardo Telles
Monthly Notices of the Royal Astronomical Society, Volume 487, Issue 4, August 2019, Pages 4669–4694
Publication year: 2019

## Summary

We present new high spectral resolution observations of 15 high-z (1.3 ≤ z ≤2.5) H II galaxies (H II G) obtained with MOSFIRE at the Keck Observatory. These data, combined with already published data for another 31 high-z and 107 z ≤0.15 H II G, are used to obtain new independent cosmological results using the distance estimator based on the established correlation between the Balmer emission line velocity dispersion and luminosity for H II G. Our results are in excellent agreement with the latest cosmological concordance model (ΛCDM) published results. From our analysis, we find a value for the mass density parameter of Ωm=0.290+0.0560.069Ωm=0.290−0.069+0.056 (stat). For a flat universe, we constrain the plane {Ωm;w0}={0.280+0.1300.100;1.12+0.580.32}{Ωm;w0}={0.280−0.100+0.130;−1.12−0.32+0.58} (stat). The joint likelihood analysis of H II G with other complementary cosmic probes (cosmic microwave background and baryon acoustic oscillations) provides tighter constraints for the parameter space of the equation of state of dark energy that are also in excellent agreement with those of similar analyses using Type Ia supernovae instead as the geometrical probe.

#### A simulated annealing algorithm to quantify patterns in astronomical data

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Maria Chira, Manolis Plionis
Monthly Notices of the Royal Astronomical Society, Volume 490, Issue 4, December 2019, Pages 5904–5920
Publication year: 2019

## Summary

We develop an optimization algorithm, using simulated annealing for the quantification of patterns in astronomical data based on techniques developed for robotic vision applications. The methodology falls in the category of cost minimization algorithms and it is based on user-determined interaction – among the pattern elements – criteria that define the properties of the sought structures. We applied the algorithm on a large variety of mock images and we constrained the free parameters; α and k, which express the amount of noise in the image and how strictly the algorithm seeks for cocircular structures, respectively. We find that the two parameters are interrelated and also that, independently of the pattern properties, an appropriate selection for most of the images would be log k = −2 and 0 < α ≲ 0.04. The width of the effective α-range, for different values of k, is reduced when more interaction coefficients are taken into account for the definition of the patterns of interest. Finally, we applied the algorithm on N-body simulation dark-matter halo data and on the HST image of the lensing Abell 2218 cluster to conclude that this versatile technique could be applied for the quantification of structure and for identifying coherence in astronomical patterns.

#### The XXL Survey: XXX. Characterisation of the XLSSsC N01 supercluster and analysis of the galaxy stellar populations

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
V. Guglielmo, B. M. Poggianti, B. Vulcani, A. Moretti, J. Fritz, F. Gastaldello, C. Adami, C. A. Caretta, J.Willis, E. Koulouridis, M. E. Ramos Ceja, P. Giles, I. Baldry, M. Birkinshaw, A. Bongiorno, M. Brown, L. Chiappetti, S. Driver, A. Elyiv, A. Evrard, M. Grootes, L. Guennou, A. Hopkins, C. Horellou, A. Iovino, S. Maurogordato, M. Owers, F. Pacaud, S. Paltani, M. Pierre, M. Plionis, T. Ponman, A. Robotham, T. Sadibekova, V. Smolčić, R. Tuffs, C. Vignali
A&A Volume 620, December 2018 The XXL Survey: second series
Publication year: 2018

## Summary

Context. Superclusters form from the largest enhancements in the primordial density perturbation field and extend for tens of Mpc, tracing the large-scale structure of the Universe. X-ray detections and systematic characterisations of superclusters and the properties of their galaxies have only been possible in the last few years.

Aims. We characterise XLSSsC N01, a rich supercluster at z ~ 0.3 detected in the XXL Survey, composed of X-ray clusters of different virial masses and X-ray luminosities. As one of the first studies on this topic, we investigate the stellar populations of galaxies in different environments in the supercluster region.

Methods. We study a magnitude-limited (r ≤ 20) and a mass-limited sample (log(M*M) ≥ 10.8) of galaxies in the virialised region and in the outskirts of 11 XLSSsC N01 clusters, in high-density field regions, and in the low-density field. We compute the stellar population properties of galaxies using spectral energy distribution (SED) and spectral fitting techniques, and study the dependence of star formation rates (SFR), colours, and stellar ages on environment.

Results. For r ≤ 20, the fraction of star-forming/blue galaxies, computed either from the specific-SFR (sSFR) or rest-frame colour, shows depletion within the cluster virial radii, where the number of galaxies with log (sSFR/ yr−1) > −12 and with (g − r)restframe < 0.6 is lower than in the field. For log(M*M) ≥ 10.8, no trends with environment emerge, as massive galaxies are mostly already passive in all environments. No differences among low- and high-density field members and cluster members emerge in the sSFR-mass relation in the mass-complete regime. Finally, the luminosity-weighted age–mass relation of the passive populations within cluster virial radii show signatures of recent environmental quenching.

Conclusions. The study of luminous and massive galaxies in this supercluster shows that while environment has a prominent role in determining the fractions of star-forming/blue galaxies, its effects on the star formation activity in star-forming galaxies are negligible.

Key words: large-scale structure of Universe / X-rays: galaxies: clusters / galaxies: groups: general / galaxies: evolution / galaxies: star formation / galaxies: stellar content

#### The XXL Survey: XXIII. The Mass Scale of XXL Clusters from Ensemble Spectroscopy

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
A. Farahi, V. Guglielmo, A. E. Evrard, B. M. Poggianti, C. Adami, S. Ettori, F. Gastaldello, P. A. Giles, B. J. Maughan, D. Rapetti, M. Sereno, B. Altieri, I. Baldry, M. Birkinshaw, M. Bolzonella, A. Bongiorno, M. Brown, L. Chiappetti, S. P. Driver, A. Elyiv, B. Garilli, L. Guennou, A. Hopkins, A. Iovino, E. Koulouridis, J. Liske, S. Maurogordato, M. Owers, F. Pacaud, M. Pierre, M. Plionis, T. Ponman, A. Robotham, T. Sadibekova, M. Scodeggio, R. Tuffs, I. Valtchanov
A&A Volume 620, December 2018 The XXL Survey: second series
Publication year: 2018

## Summary

Context. An X-ray survey with the XMM-Newton telescope, XMM-XXL, has identified hundreds of galaxy groups and clusters in two 25 deg2 fields. Combining spectroscopic and X-ray observations in one field, we determine how the kinetic energy of galaxies scales with hot gas temperature and also, by imposing prior constraints on the relative energies of galaxies and dark matter, infer a power-law scaling of total mass with temperature.

Aims. Our goals are: i) to determine parameters of the scaling between galaxy velocity dispersion and X-ray temperature, T300 kpc, for the halos hosting XXL-selected clusters, and; ii) to infer the log-mean scaling of total halo mass with temperature, ⟨lnM200 | T300 kpcz⟩.

Methods. We applied an ensemble velocity likelihood to a sample of >1500 spectroscopic redshifts within 132 spectroscopically confirmed clusters with redshifts z < 0.6 to model, ⟨lnσgal | T300 kpcz⟩, where σgal is the velocity dispersion of XXL cluster member galaxies and T300 kpc is a 300 kpc aperture temperature. To infer total halo mass we used a precise virial relation for massive halos calibrated by N-body simulations along with a single degree of freedom summarising galaxy velocity bias with respect to dark matter.

Results. For the XXL-N cluster sample, we find σgal ∝ T300 kpc0.63±0.05, a slope significantly steeper than the self-similar expectation of 0.5. Assuming scale-independent galaxy velocity bias, we infer a mean logarithmic mass at a given X-ray temperature and redshift, 〈ln(E(z)M200/1014M)|T300 kpc, z〉 = πT + αT ln (T300 kpc/Tp) + βT ln (E(z)/E(zp)) using pivot values kTp = 2.2 keV and zp = 0.25, with normalization πT = 0.45 ± 0.24 and slope αT = 1.89 ± 0.15. We obtain only weak constraints on redshift evolution, βT = −1.29 ± 1.14.

Conclusions. The ratio of specific energies in hot gas and galaxies is scale dependent. Ensemble spectroscopic analysis is a viable method to infer mean scaling relations, particularly for the numerous low mass systems with small numbers of spectroscopic members per system. Galaxy velocity bias is the dominant systematic uncertainty in dynamical mass estimates.

#### The XXL Survey XXXV. The role of cluster mass in AGN activity*

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
E. Koulouridis, M. Ricci, P. Giles, C. Adami, M. Ramos-Ceja, M. Pierre, M. Plionis, C. Lidman, I. Georgantopoulos, L. Chiappetti, A. Elyiv, S. Ettori, L. Faccioli, S. Fotopoulou, F. Gastaldello, F. Pacaud, S. Paltani, C. Vignali
A&A Volume 620, December 2018 The XXL Survey: second series
Publication year: 2018

## Summary

Context. We present the results of a study of the active galactic nucleus (AGN) density in a homogeneous and well-studied sample of 167 bona fide X-ray galaxy clusters (0.1 < z < 0.5) from the XXL Survey, from the cluster core to the outskirts (up to 6r500). The results can provide evidence of the physical mechanisms that drive AGN and galaxy evolution within clusters, testing the efficiency of ram pressure gas stripping and galaxy merging in dense environments.

Aims. The XXL cluster sample mostly comprises poor and moderately rich structures (M = 1013–4 × 1014 M), a poorly studied population that bridges the gap between optically selected groups and massive X-ray selected clusters. Our aim is to statistically study the demographics of cluster AGNs as a function of cluster mass and host galaxy position.

Methods. To investigate the effect of the environment on AGN activity, we computed the fraction of spectroscopically confirmed X-ray AGNs (LX [0.5-10 keV] > 1042 erg cm−1) in bright cluster galaxies with Mi* − 2 < M < Mi* + 1, up to 6r500 radius. The corresponding field fraction was computed from 200 mock cluster catalogues with reshuffled positions within the XXL fields. To study the mass dependence and the evolution of the AGN population, we further divided the sample into low- and high-mass clusters (below and above 1014M, respectively) and two redshift bins (0.1–0.28 and 0.28–0.5).

Results. We detect a significant excess of X-ray AGNs, at the 95% confidence level, in low-mass clusters between 0.5r500 and 2r500, which drops to the field value within the cluster cores (r < 0.5r500). In contrast, high-mass clusters present a decreasing AGN fraction towards the cluster centres, in agreement with previous studies. The high AGN fraction in the outskirts is caused by low-luminosity AGNs, up to LX [0.5-10 keV] = 1043 erg cm−1. It can be explained by a higher galaxy merging rate in low-mass clusters, where velocity dispersions are not high enough to prevent galaxy interactions and merging. Ram pressure stripping is possible in the cores of all our clusters, but probably stronger in deeper gravitational potentials. Compared with previous studies of massive or high-redshift clusters, we conclude that the AGN fraction in cluster galaxies anti-correlates strongly with cluster mass. The AGN fraction also increases with redshift, but at the same rate with the respective fraction in field galaxies.

#### The XXL Survey XXXII. Spatial clustering of the XXL-S AGN

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
M. Plionis, L.Koutoulidis, E. Koulouridis, L. Moscardini, C. Lidman, M. Pierre, C. Adami, L. Chiappetti, L. Faccioli, S. Fotopoulou, F. Pacaud, S. Paltani
A&A Volume 620, December 2018 The XXL Survey: second series
Publication year: 2018

## Summary

The XMM-XXL Survey spans two fields of 25 deg2 each observed for more than 6 Ms with XMM, which provided a sample of tens of thousands of point sources with a flux limit of ~2.2 × 10−15 and ~1.4 × 10−14 erg s−1 cm2, corresponding to 50% of the area curve, in the soft band (0.5–2 keV) and hard band (2–10 keV), respectively. In this paper we present the spatial clustering properties of ~3100 and ~1900 X-ray active galactic nuclei (AGNs) in the soft and hard bands, respectively, which have been spectroscopically observed with the AAOmega facility. This sample is 90% redshift complete down to an optical magnitude limit of r ≲ 21.8. The sources span the redshift interval 0 < z < 5.2, although in the current analysis we limit our samples to z ≤ 3, with corresponding sample median values of z̅ ≃ 0.96 and 0.79 for the soft band and hard band, respectively. We employ the projected two-point correlation function to infer the spatial clustering and find a correlation length r0 = 7.0(±0.34) and 6.42(±0.42) h−1 Mpc, respectively, for the soft- and hard-band detected sources with a slope for both cases of γ = 1.44(±0.1). The power-law clustering was detected within comoving separations of 1 and ~25 h−1 Mpc. These results, as well as those derived in two separate redshift ranges, provide bias factors of the corresponding AGN host dark matter halos that are consistent with a halo mass of log10[Mh∕(h−1M)] = 13.04 ± 0.06, confirming the results of most recent studies based on smaller X-ray AGN samples.

#### The XXL Survey XXXI. Classification and host galaxy properties of 2.1 GHz ATCA XXL-S radio sources

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Andrew Butler, Minh Huynh, Ivan Delvecchio, Anna Kapinska, Paolo Ciliegi, Nika Jurlin, Jacinta Delhaize, Vernesa Smolcic, Shantanu Desai, Sotiria Fotopoulou, Chris Lidman, Marguerite Pierre, Manolis Plionis
A&A Volume 620, December 2018 The XXL Survey: second series
Publication year: 2018

## Summary

#### The XXL Survey XXVII. The 3XLSS point source catalogue*,**

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
L. Chiappetti, S. Fotopoulou, C. Lidman, L. Faccioli, F. Pacaud, A. Elyiv, S. Paltani, M. Pierre, M. Plionis, C. Adami, S. Alis, B. Altieri, I. Baldry, M. Bolzonella, A. Bongiorno, M. Brown, S. Driver, E. Elmer, P. Franzetti, M. Grootes, V. Guglielmo, A. Iovino, E. Koulouridis, J.P. Lefevre, J. Liske, S. Maurogordato, O. Melnyk, M. Owers, B. Poggianti, M. Polletta, E. Pompei, T. Ponman, A. Robotham, T. Sadibekova, R. Tuffs, I. Valtchanov, C. Vignali, G. Wagner
A&A Volume 620, December 2018 The XXL Survey: second series
Publication year: 2018

## Summary

We present the version of the point source catalogue of the XXL Survey that was used, in part, in the first series of XXL papers. In this paper we release, in our database in Milan and at CDS: (i) the X-ray source catalogue with 26 056 objects in two areas of 25 deg2 with a flux limit (at 3σ) of ~10−15 erg s−1 cm−2 in [0.5–2] keV, and ~ 3 × 10−15 erg s−1 cm−2 in [2–10] keV, yielding a 90% completeness limit of 5.8 × 10−15 and 3.8 × 10−14 respectively;(ii) the associated multiwavelength catalogues with candidate counterparts of the X-ray sources in the infrared, near-infrared, optical, and ultraviolet (plus spectroscopic redshift when available); and (iii) a catalogue of spectroscopic redshifts recently obtained in the southern XXL area. We also present the basic properties of the X-ray point sources and their counterparts. Other catalogues described in the second series of XXL papers will be released contextually, and will constitute the second XXLdata release.

#### The XXL Survey XXV. Cosmological analysis of the C1 cluster number counts

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
F. Pacaud, M. Pierre, J.-B. Melin, C. Adami, A. E. Evrard, S. Galli, F. Gastaldello, B. J. Maughan, M. Sereno, S. Alis, B. Altieri, M. Birkinshaw, L. Chiappetti, L. Faccioli, P. A. Giles, C. Horellou, A. Iovino, E. Koulouridis, J.-P. Le Fèvre, C. Lidman, M. Lieu, S. Maurogordato, L. Moscardini, B. M. Poggianti, E. Pompei, T. Sadibekova, I. Valtchanov, J. P. Willis
A&A Volume 620, December 2018 The XXL Survey: second series
Publication year: 2018

## Summary

Context. We present an estimation of cosmological parameters with clusters of galaxies.

Aims. We constrain the Ωmσ8, and w parameters from a stand-alone sample of X-ray clusters detected in the 50 deg2 XMM-XXL survey with a well-defined selection function.

Methods. We analyse the redshift distribution of a sample comprising 178 high signal-to-noise ratio clusters out to a redshift of unity. The cluster sample scaling relations are determined in a self-consistent manner.

Results. In a lambda cold dark matter (ΛCDM) model, the cosmology favoured by the XXL clusters compares well with results derived from the Planck Sunyaev-Zel’dovich clusters for a totally different sample (mass/redshift range, selection biases, and scaling relations). However, with this preliminary sample and current mass calibration uncertainty, we find no inconsistency with the Planck CMB cosmology. If we relax the w parameter, the Planck CMB uncertainties increase by a factor of ~10 and become comparable with those from XXL clusters. Combining the two probes allows us to put constraints on Ωm = 0.316 ± 0.060, σ8 = 0.814 ± 0.054, and w = −1.02 ± 0.20.

Conclusions. This first self-consistent cosmological analysis of a sample of serendipitous XMM clusters already provides interesting insights into the constraining power of the XXL survey. Subsequent analysis will use a larger sample extending to lower confidence detections and include additional observable information, potentially improving posterior uncertainties by roughly a factor of 3.

#### The XXL Survey XXII. The XXL-North spectrophotometric sample and galaxy stellar mass function in X-ray detected groups and clusters*

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Guglielmo, V.; Poggianti, B. M.; Vulcani, B.; Adami, C.; Gastaldello, F.; Ettori, S.; Fotoupoulou, S.; Koulouridis, E.; Ramos Ceja, M. E.; Giles, P.; McGee, S.; Altieri, B.; Baldry, I.; Birkinshaw, M.; Bolzonella, M.; Bongiorno, A.; Brown, M.; Chiappetti, L.; Driver, S.; Elyiv, A. Evrard, A.; Garilli, B.; Grootes, M.; Guennou, L.; Hopkins, A.; Horellou, C.; Iovino, A.; Lidman, C.; Liske, J.; Maurogordato, S.; Owers, M.; Pacaud, F.; Paltani, S.; Pierre, M.; Plionis, M.; Ponman, T.; Robotham, A.; Sadibekova, T.; Scodeggio, M.; Sereno, M.; Smolčić, V.; Tuffs, R.; Valtchanov, I.; Vignali, C.; Willis, J.
A&A Volume 620, December 2018 The XXL Survey: second series
Publication year: 2018

## Summary

Context. The fraction of galaxies bound in groups in the nearby Universe is high (50% at z ~ 0). Systematic studies of galaxy properties in groups are important in order to improve our understanding of the evolution of galaxies and of the physical phenomena occurring within this environment.

Aims. We have built a complete spectrophotometric sample of galaxies within X-ray detected, optically spectroscopically confirmed groups and clusters (G&C), covering a wide range of halo masses at z ≤ 0.6.

Methods. In the context of the XXL survey, we analyse a sample of 164 G&C in the XXL-North region (XXL-N), at z ≤ 0.6, with a wide range of virial masses (1.24 × 1013 ≤ M500,scal(M) ≤ 6.63 × 1014) and X-ray luminosities ((2.27 × 1041 ≤ L500,scalXXL(erg s−1) ≤ 2.15 × 1044)). The G&C are X-ray selected and spectroscopically confirmed. We describe the membership assignment and the spectroscopic completeness analysis, and compute stellar masses. As a first scientific exploitation of the sample, we study the dependence of the galaxy stellar mass function (GSMF) on global environment.

Results. We present a spectrophotometric characterisation of the G&C and their galaxies. The final sample contains 132 G&C, 22 111 field galaxies and 2225 G&C galaxies with r-band magnitude <20. Of the G&C, 95% have at least three spectroscopic members, and 70% at least ten. The shape of the GSMF seems not to depend on environment (field versus G&C) or X-ray luminosity (used as a proxy for the virial mass of the system). These results are confirmed by the study of the correlation between mean stellar mass of G&C members and L500,scalXXL. We release the spectrophotometric catalogue of galaxies with all the quantities computed in this work.

Conclusions. As a first homogeneous census of galaxies within X-ray spectroscopically confirmed G&C at these redshifts, this sample will allow environmental studies of the evolution of galaxy properties.

#### The XXL survey XXI. The environment and clustering of X-ray AGN in the XXL-South field

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Melnyk, O.; Elyiv, A.; Smolčić, V.; Plionis, M.; Koulouridis, E.; Fotopoulou, S.; Chiappetti, L.; Adami, C.; Baran, N.; Butler, A.; Delhaize, J.; Delvecchio, I.; Finet, F.; Huynh, M.; Lidman, C.; Pierre, M.; Pompei, E.; Vignali, C.; Surdej, J.
A&A Volume 620, December 2018 The XXL Survey: second series
Publication year: 2018

## Summary

Context. This work is part of a series of studies focusing on the environment and the properties of the X-ray selected active galactic nuclei (AGN) population from the XXL survey. The present survey, given its large area, continuity, extensive multiwavelength coverage, and large-scale structure information, is ideal for this kind of study. Here, we focus on the XXL-South (XXL-S) field.

Aims. Our main aim is to study the environment of the various types of X-ray selected AGN and investigate its possible role in AGN triggering and evolution.

Methods. We studied the large-scale (>1 Mpc) environment up to redshift z = 1 using the nearest neighbour distance method to compare various pairs of AGN types. We also investigated the small-scale environment (<0.4 Mpc) by calculating the local overdensities of optical galaxies. In addition, we built a catalogue of AGN concentrations with two or more members using the hierarchical clustering method and we correlated them with the X-ray galaxy clusters detected in the XXL survey.

Results. It is found that radio detected X-ray sources are more obscured than non-radio ones, though not all radio sources are obscured AGN. We did not find any significant differences in the large-scale clustering between luminous and faint X-ray AGN, or between obscured and unobscured ones, or between radio and non-radio sources. At local scales (<0.4 Mpc), AGN typically reside in overdense regions, compared to non-AGN; however, no differences were found between the various types of AGN. A majority of AGN concentrations with two or more members are found in the neighbourhood of X-ray galaxy clusters within <25–45 Mpc. Our results suggest that X-ray AGN are typically located in supercluster filaments, but they are also found in over- and underdense regions.

#### The XXL Survey XX: The 365 cluster catalogue

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
C. Adami, P. Giles, E. Koulouridis, F. Pacaud, C.A. Caretta, M. Pierre, D. Eckert, M.E. Ramos-Ceja, F. Gastaldello, S. Fotopoulou, V. Guglielmo, C. Lidman, T. Sadibekova, A. Iovino, B. Maughan, L. Chiappetti, S. Alis, B. Altieri, I. Baldry, D. Bottini, M. Birkinshaw, M. Bremer, M. Brown, O. Cucciati, S. Driver, E. Elmer, S. Ettori, A.E. Evrard, L. Faccioli, B. Granett, M. Grootes, L. Guzzo, A. Hopkins, C. Horellou, J.P. Lef, J. Liske, K. Malek, F. Marulli, S. Maurogordato, M. Owers, S. Paltani, B. Poggianti, M. Polletta, M. Plionis, A. Pollo, E. Pompei, T. Ponman, D. Rapetti, M. Ricci, A. Robotham, R. Tuffs, L. Tasca, I. Valtchanov, D. Vergani, G. Wagner, J. Willis, the XXL consortium
A&A Volume 620, December 2018 The XXL Survey: second series
Publication year: 2018

## Summary

Context. In the currently debated context of using clusters of galaxies as cosmological probes, the need for well-defined cluster samples is critical.

Aims. The XXL Survey has been specifically designed to provide a well characterised sample of some 500 X-ray detected clusters suitable for cosmological studies. The main goal of present article is to make public and describe the properties of the cluster catalogue in its present state, as well as of associated catalogues of more specific objects such as super-clusters and fossil groups.

Methods. Following from the publication of the hundred brightest XXL clusters, we now release a sample containing 365 clusters in total, down to a flux of a few 10−15 erg s−1 cm−2 in the [0.5–2] keV band and in a 1′ aperture. This release contains the complete subset of clusters for which the selection function is well determined plus all X-ray clusters which are, to date, spectroscopically confirmed. In this paper, we give the details of the follow-up observations and explain the procedure adopted to validate the cluster spectroscopic redshifts. Considering the whole XXL cluster sample, we have provided two types of selection, both complete in a particular sense: one based on flux-morphology criteria, and an alternative based on the [0.5–2] keV flux within 1 arcmin of the cluster centre. We have also provided X-ray temperature measurements for 80% of the clusters having a flux larger than 9 × 10−15 erg s−1 cm−2.

Results. Our cluster sample extends from z ~ 0 to z ~ 1.2, with one cluster at z ~ 2. Clusters were identified through a mean number of six spectroscopically confirmed cluster members. The largest number of confirmed spectroscopic members in a cluster is 41. Our updated luminosity function and luminosity–temperature relation are compatible with our previous determinations based on the 100 brightest clusters, but show smaller uncertainties. We also present an enlarged list of super-clusters and a sample of 18 possible fossil groups.

Conclusions. This intermediate publication is the last before the final release of the complete XXL cluster catalogue when the ongoing C2 cluster spectroscopic follow-up is complete. It provides a unique inventory of medium-mass clusters over a 50 deg2 area out to z ~ 1.

#### The XXL Survey XVI. The clustering of X-ray selected galaxy clusters at z ~ 0.3

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
F. Marulli, A. Veropalumbo, M. Sereno, L. Moscardini, F. Pacaud, M. Pierre, M. Plionis, A. Cappi, C. Adami, S. Alis, B. Altieri, M. Birkinshaw, S. Ettori, L. Faccioli, F. Gastaldello, E. Koulouridis, C. Lidman, J.-P. Le Fèvre, S. Maurogordato, B. Poggianti, E. Pompei, T. Sadibekova, and I. Valtchanov
A&A Volume 620, December 2018 The XXL Survey: second series
Publication year: 2018

## Summary

Context. Galaxy clusters trace the highest density peaks in the large-scale structure of the Universe. Their clustering provides a powerful probe that can be exploited in combination with cluster mass measurements to strengthen the cosmological constraints provided by cluster number counts.

Aims. We investigate the spatial properties of a homogeneous sample of X-ray selected galaxy clusters from the XXL survey, the largest programme carried out by the XMM-Newton satellite. The measurements are compared to Λ-cold dark matter predictions, and used in combination with self-calibrated mass scaling relations to constrain the effective bias of the sample, beff, and the matter density contrast, ΩM.

Methods. We measured the angle-averaged two-point correlation function of the XXL cluster sample. The analysed catalogue consists of 182 X-ray selected clusters from the XXL second data release, with median redshift ⟨z⟩ = 0.317 and median mass ⟨M500⟩≃ 1.3 × 1014M. A Markov chain Monte Carlo analysis is performed to extract cosmological constraints using a likelihood function constructed to be independent of the cluster selection function.

Results. Modelling the redshift-space clustering in the scale range 10 < r [h−1 Mpc] < 40, we obtain ΩM = 0.27−0.04+0.06 and beff = 2.73−0.20+0.18.

This is the first time the two-point correlation function of an X-ray selected cluster catalogue at such relatively high redshifts and low masses has been measured. The XXL cluster clustering appears fully consistent with standard cosmological predictions. The analysis presented in this work demonstrates the feasibility of a cosmological exploitation of the XXL cluster clustering, paving the way for a combined analysis of XXL cluster number counts and clustering.

#### The XXL Survey XIX. A realistic population of simulated X-ray AGN: Comparison of models with observations

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Koulouridis, E.; Faccioli, L.; Le Brun, A. M. C.; Plionis, M.; McCarthy, I. G.; Pierre, M.; Akylas, A.; Georgantopoulos, I.; Paltani, S.; Lidman, C.; Fotopoulou, S.; Vignali, C.; Pacaud, F.; Ranalli, P.
A&A Volume 620, December 2018 The XXL Survey: second series
Publication year: 2018

## Summary

Modern cosmological simulations heavily rely on feedback from active galactic nuclei (AGN) in order to stave off overcooling in massive galaxies, and galaxy groups and clusters. Given that AGN are a key component of such simulations, an important independent test is whether or not the simulations capture the broad demographics of the observed AGN population. However, to date, comparisons between observed and simulated AGN populations have been relatively limited. Here, we have used the cosmo-OWLS suite of cosmological hydrodynamical simulations to produce realistic synthetic catalogs of X-ray AGN out to z = 3, with the aim of comparing the catalogs to the observed X-ray AGN population in the XXL survey and other recent surveys. We focused on the unabsorbed X-ray luminosity function (XLF), the Eddington ratio distribution, the black hole mass function, and the projected clustering of X-ray AGN. To compute the unabsorbed XLF of the simulated AGN, we used recent empirically-determined (luminosity-dependent) bolometric corrections, in order to convert the simulated bolometric luminosity into an observable X-ray luminosity. We show that, using these corrections, the simulated AGN sample accurately reproduces the observed XLF over 3 orders of magnitude in X-ray luminosity in all redshift bins from z = 0 out to z = 3. To compare to the observed Eddington ratio distribution and the clustering of AGN, we produced detailed “XMM-Newton-detected” catalogs of the simulated AGN. This requires the production of synthetic X-ray images extracted from light cones of the simulations, which self-consistently contain both the X-ray AGN and the emission from diffuse, hot gas within galaxies, galaxy groups, and clusters and that fold in the relevant instrumental effects of XMM-Newton. We apply a luminosity- and redshift-dependent obscuration function for the AGN and employ the same AGN detection algorithm as used for the real XXL survey. We demonstrate that the detected population of simulated AGN reproduces the observed Eddington ratio distribution and projected clustering from XXL quite well. Based on these comparisons, we conclude that the simulations have a broadly realistic population of AGN and that our synthetic X-ray AGN catalogs should be useful for interpreting additional trends (e.g. environmental dependencies) and as a helpful tool for quantifying AGN contamination in galaxy group and cluster X-ray surveys.

#### Disentangling the AGN and Star-Formation connection using XMM-Newton

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
V. A. Masoura, G. Mountrichas, I. Georgantopoulos, A. Ruiz, G. Magdis, M. Plionis
A&A Volume 618, October 2018
Publication year: 2018

## Summary

There is growing evidence supporting the coeval growth of galaxies and their resident super-massive black hole (SMBH). Most studies also claim a correlation between the activity of the SMBH and the star formation of the host galaxy. It is unclear, however, whether this correlation extends to all redshifts and X-ray luminosities. Some studies find a weaker dependence at lower luminosities and/or a suppression of the star formation at high luminosities. We here use data from the X-ATLAS and XMM-XXL North fields and compile the largest X-ray sample up to date to investigate how X-ray selected AGN affect the star formation of their host galaxies in a wide redshift and luminosity baseline of 0.03 < z < 3 and log LX(2−10 keV) = (41−45.5) erg s−1. Our sample consists of 3336 AGN. 1872 of our sources have spectroscopic redshifts. For the remaining sources we calculate photometric redshifts using TPZ, a machine-learning algorithm. We estimate stellar masses (M) and star formation rates (SFRs) by applying spectral energy distribution fitting through the CIGALE code, using optical, near-IR, and mid-IR photometry (SDSS, VISTA, and WISE). Of our sources, 608 also have far-IR photometry (Herschel). We use these sources to calibrate the SFR calculations of our remaining X-ray sample. Our results show a correlation between the X-ray luminosity (LX) and the SFR of the host galaxy at all redshifts and luminosities spanned by our sample. We also find a dependence of the specific SFR (sSFR) on redshift, while there are indications that the X-ray luminosity enhances the sSFR even at low redshifts. We then disentangle the effects of stellar mass and redshift on the SFR and again study its dependence on the X-ray luminosity. Towards this end, we estimate the SFR of main-sequence galaxies that have the same stellar mass and redshift as our X-ray AGN and compare them with the SFR of our X-ray AGN. Our analysis reveals that the AGN enhances the star formation of its host galaxy when the galaxy lies below the main sequence and quenches the star formation of the galaxy it lives in when the host lies above the main sequence. Therefore, the effect of AGN on the SFR of the host galaxy depends on the location of the galaxy relative to the main sequence.

#### Dependence on the environment of the abundance function of light-cone simulation dark matter haloes

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Maria Chira, Manolis Plionis, and Pier-Stefano Corasaniti
A&A Volume 616, August 2018
Publication year: 2018

## Summary

Aims. We study the dependence of the halo abundance function (AF) on different environments in a whole-sky ΛCDM light-cone halo catalogue extending to z ~ 0.65, using a simple and well-defined halo isolation criterion.

Methods. The isolation status of each individual dark matter halo is determined by the distance to its nearest neighbour, which defines the maximum spherical region devoid of halos above a threshold mass around it (although the true size of such region may be much larger since it is not necessarily spherical). A versatile double power-law Schechter function is used to fit the dark matter halo AF, and its derived parameters are studied as a function of halo isolation status.

Results. (a) Our function fits the halo abundances for all halo isolation statuses extremely well, while the well-established theoretical mass functions, integrated over the volume of the light-cone, provide an adequate but poorer fit than our phenomenological model. (b) As expected, and in agreement with other studies based on snap-shot simulations, we find significant differences of the halo abundance function as a function of halo isolation, indicating different rates of halo formation. The slope of the power law and the characteristic mass of the Schechter-like fitting function decrease with isolation, a result consistent with the formation of less massive haloes in lower density regions. (c) We find an unexpected upturn of the characteristic mass of the most isolated haloes of our sample. This upturn originates and characterises only the higher redshift regime (z ≳ 0.45), which probably implies a significant and recent evolution of the isolation status of the most isolated and most massive haloes.

#### Dependence of clustering of X-ray AGN on obscuration

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
L Koutoulidis, I Georgantopoulos, G Mountrichas, M Plionis, A Georgakakis, A Akylas, E Rovilos
Monthly Notices of the Royal Astronomical Society, Volume 481, Issue 3, December 2018, Pages 3063–3069
Publication year: 2018

## Summary

Recent studies which select active galactic nucleus (AGN) in the mid-infrared (IR) part of the spectrum find that obscured AGNs reside in more massive dark matter haloes compared to unobscured ones. In contrast, X-ray AGN surveys do not find a difference in the dark matter haloes of these two populations. We visit anew this issue by examining the clustering properties of a large X-ray sample distributed over five deep fields. These are the CDF-N, CDF-S, ECDF-S, COSMOS, and AEGIS Chandra fields spanning the redshift interval 0.6 < zz < 1.4. In particular, we present the clustering properties of 736 and 720 unobscured and obscured X-ray-selected AGNs (0.5–8 keV) with column densities higher and lower than NH=1022cm2NH=1022cm−2, respectively. We perform a spatial correlation function analysis for the two samples, and we find a weak (2σ) difference in the clustering of obscured sources (ro=7.0±0.6ro=7.0±0.6h−1 Mpc) compared to that of unobscured sources (ro=5.4±0.6ro=5.4±0.6h−1 Mpc) using a fixed slope of γ = 1.8. Furthermore, we compare our findings with recent results that base the obscured and unobscured AGN classification on the optical/IR colour (R[4.5]=6.1R−[4.5]=6.1). We find that the optical/IR criterion fails to identify a purely AGN sample. In particular, reddened AGNs with R[4.5]>6.1R−[4.5]>6.1 are divided almost equally between X-ray obscured and unobscured AGNs. Derivation of the spectral energy distributions reveals that in many cases the host galaxy contaminates the mid-IR bands thus affecting the optical/mid-IR obscured AGN classification.

#### Comparison of the linear bias models in the light of the Dark Energy Survey

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
A Papageorgiou, S Basilakos, M Plionis
Monthly Notices of the Royal Astronomical Society, Volume 476, Issue 2, May 2018, Pages 2621–2628
Publication year: 2018

## Summary

The evolution of the linear and scale independent bias, based on the most popular dark matter bias models within the Λ cold dark matter (ΛCDM) cosmology, is confronted to that of the Dark Energy Survey (DES) luminous red galaxies (LRGs). Applying a χ2 minimization procedure between models and data, we find that all the considered linear bias models reproduce well the LRG bias data. The differences among the bias models are absorbed in the predicted mass of the dark-matter halo in which LRGs live and which ranges between ∼6 × 1012 and 1.4 × 1013h−1 M, for the different bias models. Similar results, reaching however a maximum value of ∼2 × 1013h−1 M, are found by confronting the SDSS (2SLAQ) Large Red Galaxies clustering with theoretical clustering models, which also include the evolution of bias. This later analysis also provides a value of Ωm = 0.30 ± 0.01, which is in excellent agreement with recent joint analyses of different cosmological probes and the reanalysis of the Planck data.

#### AzTEC 1.1 mm observations of high-z protocluster environments: SMG overdensities and misalignment between AGN jets and SMG distribution

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
M Zeballos, I Aretxaga, D H Hughes, A Humphrey, G W Wilson, J Austermann, J S Dunlop, H Ezawa, D Ferrusca, B Hatsukade, R J Ivison, R Kawabe, S Kim, T Kodama, K Kohno, A Montaña, K Nakanishi, M Plionis, D Sánchez-Argüelles, J A Stevens, Y Tamura, M Velazquez, M S Yun
Monthly Notices of the Royal Astronomical Society, Volume 479, Issue 4, October 2018, Pages 4577–4632
Publication year: 2018

## Summary

We present observations at 1.1 mm towards 16 powerful radio galaxies and a radio-quiet quasar at 0.5 < z < 6.3 acquired with the AzTEC camera mounted at the James Clerk Maxwell Telescope and Atacama Submillimeter Telescope Experiment to study the spatial distribution of submillimetre galaxies (SMGs) towards possible protocluster regions. The survey covers a total area of 1.01 sq deg with rms depths of 0.52–1.44 mJy and detects 728 sources above 3σ. We find overdensities of a factor of ∼2 in the source counts of three individual fields (4C+23.56, PKS1138−262, and MRC0355−037) over areas of ∼200 sq deg. When combining all fields, the source-count analysis finds an overdensity that reaches a factor ≳3 at S1.1mm ≥ 4 mJy covering a 1.5-arcmin-radius area centred on the active galactic nucleus. The large size of our maps allows us to establish that beyond a radius of 1.5 arcmin, the radial surface density of SMGs falls to that of a blank field. In addition, we find a trend for SMGs to align closely to a perpendicular direction with respect to the radio jets of the powerful central radio galaxies (7314+13+13−14 deg). This misalignment is found over projected comoving scales of 4–20 Mpc, departs from perfect alignment (0 deg) by ∼5σ, and apparently has no dependence on SMG luminosity. Under the assumption that the AzTEC sources are at the redshift of the central radio galaxy, the misalignment reported here can be interpreted as SMGs preferentially inhabiting mass-dominant filaments funnelling material towards the protoclusters, which are also the parent structures of the radio galaxies.

#### An independent determination of the local Hubble constant

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
David Fernández Arenas, Elena Terlevich, Roberto Terlevich, Jorge Melnick, Ricardo Chávez, Fabio Bresolin, Eduardo Telles, Manolis Plionis, Spyros Basilakos
Monthly Notices of the Royal Astronomical Society, Volume 474, Issue 1, February 2018, Pages 1250–1276
Publication year: 2018

## Summary

The relationship between the integrated H β line luminosity and the velocity dispersion of the ionized gas of H II galaxies and giant H II regions represents an exciting standard candle that presently can be used up to redshifts z ∼ 4. Locally it is used to obtain precise measurements of the Hubble constant by combining the slope of the relation obtained from nearby (z ≤ 0.2) H II galaxies with the zero-point determined from giant H II regions belonging to an ‘anchor sample’ of galaxies for which accurate redshift-independent distance moduli are available. We present new data for 36 giant H II regions in 13 galaxies of the anchor sample that includes the megamaser galaxy NGC 4258. Our data are the result of the first 4 yr of observation of our primary sample of 130 giant H II regions in 73 galaxies with Cepheid determined distances. Our best estimate of the Hubble parameter is 71.0 ± 2.8(random) ± 2.1(systematic) km s− 1Mpc− 1. This result is the product of an independent approach and, although at present less precise than the latest SNIa results, it is amenable to substantial improvement.

#### The XXL survey: First results and future

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
M. Pierre, C. Adami, M. Birkinshaw, L. Chiappetti, S. Ettori, A. Evrard, L. Faccioli, F. Gastaldello, P. Giles, C. Horellou, A. Iovino, E. Koulouridis, C. Lidman, A. Le Brun, B. Maughan, S. Maurogordato, I. McCarthy, S. Miyazaki, F. Pacaud, S. Paltani, M. Plionis, T. Reiprich, T. Sadibekova, V. Smolcic, S. Snowden, J. Surdej, M. Tsirou, C. Vignali, J. Willis, S. Alis, B. Altieri, N. Baran, C. Benoist, A. Bongiorno, M. Bremer, A. Butler, A. Cappi, C. Caretta, P. Ciliegi, N. Clerc, P. S. Corasaniti, J. Coupon, J. Delhaize, I. Delvecchio, J. Democles, Sh. Desai, J. Devriendt, Y. Dubois, D. Eckert, A. Elyiv, A. Farahi, C. Ferrari, S. Fotopoulou, W. Forman, I. Georgantopoulos, V. Guglielmo, M. Huynh, N. Jerlin, Ch. Jones, S. Lavoie, J.-P. Le Fevre, M. Lieu, M. Kilbinger, F. Marulli, A. Mantz, S. McGee, J.-B. Melin, O. Melnyk, L. Moscardini, M. Novak, E. Piconcelli, B. Poggianti, D. Pomarede, E. Pompei, T. Ponman, M. E. Ramos Ceja, P. Ranalli, D. Rapetti, S. Raychaudhury, M. Ricci, H. Rottgering, M. Sahlén, J.-L. Sauvageot, C. Schimd, M. Sereno, G.P. Smith, K. Umetsu, P. Valageas, A. Valotti, I. Valtchanov, A. Veropalumbo, B. Ascaso, D. Barnes, M. De Petris, F. Durret, M. Donahue, M. Ithana, M. Jarvis, M. Johnston-Hollitt, E. Kalfountzou , S. Kay, F. La Franca, N. Okabe, A. Muzzin, A. Rettura, F. Ricci, J. Ridl, G. Risaliti, M. Takizawa, P. Thomas, N. Truong
Astron. Nachr. / AN. 2017;338:334–341
Publication year: 2017

## Summary

The XXL survey currently covers two 25 deg2 patches with XMM observations of ∼10 ks. We summarize the scientific results associated with the first release of the XXL dataset, which occurred in mid‐2016. We review several arguments for increasing the survey depth to 40 ks during the next decade of XMM operations. X‐ray (z < 2) cluster, (z < 4) active galactic nuclei (AGN), and cosmic background survey science will then benefit from an extraordinary data reservoir. This, combined with deep multi‐λ observations, will lead to solid standalone cosmological constraints and provide a wealth of information on the formation and evolution of AGN, clusters, and the X‐ray background. In particular, it will offer a unique opportunity to pinpoint the z > 1 cluster density. It will eventually constitute a reference study and an ideal calibration field for the upcoming eROSITA and Euclid missions.

#### The L−σ relation for HII galaxies in green

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Melnick J., Telles E., Bordalo V., Chávez R., Fernández-Arenas D., Terlevich E., Terlevich R., Bresolin, F. Plionis M., Basilakos S.
Astronomy & Astrophysics, Volume 599, id.A76, 13 pp.
Publication year: 2017

## Summary

The correlation between emission-line luminosity (L) and profile-width (σ) for HII galaxies provides a powerful method to measure the distances to galaxies over a wide range of redshifts. In this paper, we use SDSS spectrophotometry to explore the systematics of the correlation using the [OIII]5007 lines instead of Hα or Hβ to measure luminosities and line widths. We also examine possible systematic effects involved in measuring the profile-widths and the luminosities through different apertures. We find that the green Lσ relation, defined using [OIII]5007 luminosities, is significantly more sensitive than Hβ to the effects of age and the physical conditions of the nebulae, which more than offsets the advantage of the higher strength of the [OIII]5007 lines. We then explore the possibility of mixing [OIII]5007 profile-widths with SDSS Hβ luminosities using the Hubble constant H0 to quantify the possible systematic effects. We find the mixed L(Hβ− σ[OIII] relation to be at least as powerful as the canonical Lσ relation as a distance estimator, and we show that evolutionary corrections do not change the slope and the scatter of the correlation and, therefore, do not bias the Lσ distance indicator at high redshifts. Locally, however, the luminosities of the giant HII regions that provide the zero-point calibrators are sensitive to evolutionary corrections and may bias the Hubble constant if their mean ages, as measured by the equivalent widths of Hβ, are significantly different from the mean age of the HII galaxies. Using a small sample of 16 ad-hoc zero point calibrators we obtain a value of H0 = 66.4+ 5.0-4.5km s-1 Mpc-1 for the Hubble constant, which is fully consistent with the best modern determinations, and which is not biased by evolutionary corrections.

#### Galaxy cluster’s rotation

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
M. Manolopoulou, M. Plionis
Monthly Notices of the Royal Astronomical Society, Volume 465, Issue 3, March 2017, Pages 2616–2633
Publication year: 2017

## Summary

We study the possible rotation of cluster galaxies, developing, testing, and applying a novel algorithm which identifies rotation, if such does exist, as well as its rotational centre, its axis orientation, rotational velocity amplitude, and, finally, the clockwise or counterclockwise direction of rotation on the plane of the sky. To validate our algorithms we construct realistic Monte Carlo mock rotating clusters and confirm that our method provides robust indications of rotation. We then apply our methodology on a sample of Abell clusters with z ≲ 0.1 with member galaxies selected from the Sloan Digital Sky Survey DR10 spectroscopic data base. After excluding a number of substructured clusters, which could provide erroneous indications of rotation, and taking into account the expected fraction of misidentified coherent substructure velocities for rotation, provided by our Monte Carlo simulation analysis, we find that ∼23 per cent of our clusters are rotating under a set of strict criteria. Loosening the strictness of the criteria, on the expense of introducing spurious rotation indications, we find this fraction increasing to ∼28 per cent. We correlate our rotation indicators with the cluster dynamical state, provided either by their Bautz–Morgan type or by their X-ray isophotal shape and find for those clusters showing rotation within 1.5 h170h70−1 Mpc that the significance of their rotation is related to the dynamically younger phases of cluster formation but after the initial anisotropic accretion and merging has been completed. Finally, finding rotational modes in galaxy clusters could lead to the necessity of correcting the dynamical cluster mass calculations.

#### The XXL Survey⋆,⋆⋆ XII. Optical spectroscopy of X-ray-selected clusters and the frequency of AGN in superclusters

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
E. Koulouridis, B. Poggianti, B. Altieri, I. Valtchanov, Y. Jaffé, C. Adami, A. Elyiv, O. Melnyk, S. Fotopoulou, F. Gastaldello, C. Horellou, M. Pierre, F. Pacaud, M. Plionis, T. Sadibekova and J. Surdej
A&A Volume 592, August 2016 The XXL Survey: First results
Publication year: 2016

## Summary

Context. This article belongs to the first series of XXL publications. It presents multifibre spectroscopic observations of three 0.55 deg2 fields in the XXL Survey, which were selected on the basis of their high density of X-ray-detected clusters. The observations were obtained with the AutoFib2+WYFFOS (AF2) wide-field fibre spectrograph mounted on the 4.2 m William Herschel Telescope.

Aims. The paper first describes the scientific rationale, the preparation, the data reduction, and the results of the observations, and then presents a study of active galactic nuclei (AGN) within three superclusters.

Methods. To determine the redshift of galaxy clusters and AGN, we assign high priority to a) the brightest cluster galaxies (BCGs), b) the most probable cluster galaxy candidates, and c) the optical counterparts of X-ray point-like sources. We use the outcome of the observations to study the projected (2D) and the spatial (3D) overdensity of AGN in three superclusters.

Results. We obtained redshifts for 455 galaxies in total, 56 of which are counterparts of X-ray point-like sources. We were able to determine the redshift of the merging supercluster XLSSC-e, which consists of six individual clusters at z ~ 0.43, and we confirmed the redshift of supercluster XLSSC-d at z ~ 0.3. More importantly, we discovered a new supercluster, XLSSC-f, that comprises three galaxy clusters also at z ~ 0.3. We find a significant 2D overdensity of X-ray point-like sources only around the supercluster XLSSC-f. This result is also supported by the spatial (3D) analysis of XLSSC-f, where we find four AGN with compatible spectroscopic redshifts and possibly one more with compatible photometric redshift. In addition, we find two AGN (3D analysis) at the redshift of XLSSC-e, but no AGN in XLSSC-d. Comparing these findings with the optical galaxy overdensity we conclude that the total number of AGN in the area of the three superclusters significantly exceeds the field expectations. All of the AGN found have luminosities below 7 × 1042 erg s-1.

Conclusions. The difference in the AGN frequency between the three superclusters cannot be explained by the present study because of small number statistics. Further analysis of a larger number of superclusters within the 50 deg2 of the XXL is needed before any conclusions on the effect of the supercluster environment on AGN can be reached.

#### The XXL Survey⋆,⋆⋆ I. Scientific motivations − XMM-Newton observing plan − Follow-up observations and simulation programme

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
M. Pierre, F. Pacaud, C. Adami, S. Alis, B. Altieri, N. Baran, C. Benoist, M. Birkinshaw, A. Bongiorno, M. N. Bremer, M. Brusa, A. Butler, P. Ciliegi, L. Chiappetti, N. Clerc, P. S. Corasaniti, J. Coupon, C. De Breuck, J. Democles, S. Desai, J. Delhaize, J. Devriendt, Y. Dubois, , D. Eckert, A. Elyiv, S. Ettori, , A. Evrard, L. Faccioli, A. Farahi, C. Ferrari, F. Finet, , S. Fotopoulou, N. Fourmanoit, P. Gandhi, F. Gastaldello, R. Gastaud, I. Georgantopoulos, P. Giles, L. Guennou, V. Guglielmo, C. Horellou, K. Husband, M. Huynh, A. Iovino, M. Kilbinger, E. Koulouridis, S. Lavoie, A. M. C. Le Brun, ,J. P. Le Fevre, C. Lidman, M. Lieu, C. A. Lin, A. Mantz, B. J. Maughan, S. Maurogordato, I. G. McCarthy, S. McGee, J. B. Melin, O. Melnyk, F. Menanteau, M. Novak, S. Paltani, M. Plionis, B. M. Poggianti, D. Pomarede, E. Pompei, T. J. Ponman, M. E. Ramos-Ceja, P. Ranalli, D. Rapetti, S. Raychaudury, T. H. Reiprich, H. Rottgering, E. Rozo, E. Rykoff, T. Sadibekova, J. Santos, J. L. Sauvageot, C. Schimd, M. Sereno, G. P. Smith, S. Snowden, D. Spergel, S. Stanford, J. Surdej, P. Valageas, A. Valotti, I. Valtchanov, C. Vignali,V. Smolčić, J. Willis and F. Ziparo
A&A Volume 592, August 2016 The XXL Survey: First results
Publication year: 2016

## Summary

Context. The quest for the cosmological parameters that describe our universe continues to motivate the scientific community to undertake very large survey initiatives across the electromagnetic spectrum. Over the past two decades, the Chandra and XMM-Newton observatories have supported numerous studies of X-ray-selected clusters of galaxies, active galactic nuclei (AGNs), and the X-ray background. The present paper is the first in a series reporting results of the XXL-XMM survey; it comes at a time when the Planck mission results are being finalised.

Aims. We present the XXL Survey, the largest XMM programme totaling some 6.9 Ms to date and involving an international consortium of roughly 100 members. The XXL Survey covers two extragalactic areas of 25 deg2 each at a point-source sensitivity of ~5 × 10-15 erg s-1 cm-2 in the [0.5−2] keV band (completeness limit). The survey’s main goals are to provide constraints on the dark energy equation of state from the space-time distribution of clusters of galaxies and to serve as a pathfinder for future, wide-area X-ray missions. We review science objectives, including cluster studies, AGN evolution, and large-scale structure, that are being conducted with the support of approximately 30 follow-up programmes.

Methods. We describe the 542 XMM observations along with the associated multi-λ and numerical simulation programmes. We give a detailed account of the X-ray processing steps and describe innovative tools being developed for the cosmological analysis.

Results. The paper provides a thorough evaluation of the X-ray data, including quality controls, photon statistics, exposure and background maps, and sky coverage. Source catalogue construction and multi-λ associations are briefly described. This material will be the basis for the calculation of the cluster and AGN selection functions, critical elements of the cosmological and science analyses.

Conclusions. The XXL multi-λ data set will have a unique lasting legacy value for cosmological and extragalactic studies and will serve as a calibration resource for future dark energy studies with clusters and other X-ray selected sources. With the present article, we release the XMM XXL photon and smoothed images along with the corresponding exposure maps.

#### The XXL Survey XIV. AAOmega Redshifts for the Southern XXL Field

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
C. Lidman, F. Ardila, M. Owers,, C. Adami, L. Chiappetti, F. Civano, A. Elyiv, F. Finet,S. Fotopoulou, A. Goulding, E. Koulouridis, O. Melnyk, F. Menanteau, F. Pacaud,M. Pierre, M. Plionis, J. Surdej and T. Sadibekova
Publications of the Astronomical Society of Australia (PASA), Vol. 33, e001, 7 pages (2016).
Publication year: 2016

## Summary

We present a catalogue containing the redshifts of 3 660 X-ray selected targets in the XXL southern field. The redshifts were obtained with the AAOmega spectrograph and 2dF fibre positioner on the Anglo-Australian Telescope. The catalogue contains 1 515 broad line AGN, 528 stars, and redshifts for 41 out of the 49 brightest X-ray selected clusters in the XXL southern field.

#### The XXL Survey VI. The 1000 brightest X-ray point sources⋆,⋆⋆

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Fotopoulou, F. Pacaud, S. Paltani, P. Ranalli, M. E. Ramos-Ceja, L. Faccioli, M. Plionis, C. Adami, A. Bongiorno, M. Brusa, L. Chiappetti, S. Desai, A. Elyiv, C. Lidman, O. Melnyk, M. Pierre, E. Piconcelli9, C. Vignali, S. Alis, F. Ardila, S. Arnouts, I. Baldry, M. Bremer, D. Eckert, L. Guennou, C. Horellou, A. Iovino, E. Koulouridis, J. Liske, S. Maurogordato, F. Menanteau, J. J. Mohr, M. Owers, B. Poggianti, E. Pompei, T. Sadibekova, A. Stanford, R. Tuffs and J. Willis
A&A Volume 592, August 2016 The XXL Survey: First results
Publication year: 2016

## Summary

Context. X-ray extragalactic surveys are ideal laboratories for the study of the evolution and clustering of active galactic nuclei (AGN). Usually, a combination of deep and wide surveys is necessary to create a complete picture of the population. Deep X-ray surveys provide the faint population at high redshift, while wide surveys provide the rare bright sources. Nevertheless, very wide area surveys often lack the ancillary information available for modern deep surveys. The XXL survey spans two fields of a combined 50 deg2 observed for more than 6Ms with XMM-Newton, occupying the parameter space that lies between deep surveys and very wide area surveys; at the same time it benefits from a wealth of ancillary data.

Aims. This paper marks the first release of the XXL point source catalogue including four optical photometry bands and redshift estimates. Our sample is selected in the 2 − 10 keV energy band with the goal of providing a sizable sample useful for AGN studies. The limiting flux is F2 − 10 keV = 4.8 × 10-14 erg s-1 cm-2.

Methods. We use both public and proprietary data sets to identify the counterparts of the X-ray point-like sources by means of a likelihood ratio test. We improve upon the photometric redshift determination for AGN by applying a Random Forest classification trained to identify for each object the optimal photometric redshift category (passive, star forming, starburst, AGN, quasi-stellar objects (QSO)). Additionally, we assign a probability to each source that indicates whether it might be a star or an outlier. We apply Bayesian analysis to model the X-ray spectra assuming a power-law model with the presence of an absorbing medium.

Results. We find that the average unabsorbed photon index is ⟨Γ⟩ = 1.85 ± 0.40 while the average hydrogen column density is log ⟨NH⟩ = 21.07 ± 1.2 cm-2. We find no trend of Γ or NH with redshift and a fraction of 26% absorbed sources (log NH> 22) consistent with the literature on bright sources (log Lx> 44). The counterpart identification rate reaches 96.7% for sources in the northern field, 97.7% for the southern field, and 97.2% in total. The photometric redshift accuracy is 0.095 for the full XMM-XXL with 28% catastrophic outliers estimated on a sample of 339 sources.

Conclusions. We show that the XXL-1000-AGN sample number counts extended the number counts of the COSMOS survey to higher fluxes and are fully consistent with the Euclidean expectation. We constrain the intrinsic luminosity function of AGN in the 2 − 10 keV energy band where the unabsorbed X-ray flux is estimated from the X-ray spectral fit up to z = 3. Finally, we demonstrate the presence of a supercluster size structure at redshift 0.14, identified by means of percolation analysis of the XXL-1000-AGN sample. The XXL survey, reaching a medium flux limit and covering a wide area, is a stepping stone between current deep fields and planned wide area surveys.

#### Testing the isotropy of the Hubble expansion

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
K. Migkas, M. Plionis
Revista Mexicana de Astronom´ıa y Astrof´ısica, 52, 133–141 (2016)
Publication year: 2016

## Summary

We have used the Union2.1 SNIa compilation to search for possible Hubble expansion anisotropies, dividing the sky in 9 solid angles containing roughly the same number of SNIa, as well as in two Galactic hemispheres. We identified only one sky region, containing 82 SNIa (~15% of total sample with z > 0.02), that indeed appears to share a Hubble expansion significantly different from the rest of the sample. However, this behaviour can be attributed to the joint “erratic” behaviour of only three SNIa and not to an anisotropic expansion. We also find that the northern and southern galactic hemispheres have different cosmological parameter solutions, but still not significant enough to support a Hubble expansion anisotropy. We conclude that even a few outliers can induce artificial indications of anisotropies, when the number of analysed SNIa is relatively small.

#### Direct measurement of lensing amplification in Abell S1063 using a strongly lensed high redshift HII galaxy

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Roberto Terlevich, Jorge Melnick, Elena Terlevich, Ricardo Chávez, Eduardo Telles, Fabio Bresolin, Manolis Plionis, Spyros Basilakos, David Fernández Arenas, Ana Luisa González Morán, Ángeles I. Díaz and Itziar Aretxaga
A&A Volume 592, August 2016
Publication year: 2016

## Summary

ID11 is an actively star-forming, extremely compact galaxy and Lyα emitter at z = 3.117 that is gravitationally magnified by a factor of ~17 by the cluster of galaxies Hubble Frontier Fields AS1063. The observed properties of this galaxy resemble those of low luminosity HII galaxies or giant HII regions such as 30 Doradus in the Large Magellanic Cloud. Using the tight correlation correlation between the Balmer-line luminosities and the width of the emission lines (typically L(Hβ) − σ(Hβ)), which are valid for HII galaxies and giant HII regions to estimate their total luminosity, we are able to measure the lensing amplification of ID11. We obtain an amplification of 23 ± 11 that is similar within errors to the value of ~17 estimated or predicted by the best lensing models of the massive cluster Abell S1063. We also compiled, from the literature, luminosities and velocity dispersions for a set of lensed compact star-forming regions. There is more scatter in the Lσ correlation for these lensed systems, but on the whole the results tend to support the lensing model estimates of the magnification. Our result indicates that the amplification can be independently measured using the L − σ relation in lensed giant HII regions or HII galaxies. It also supports the suggestion, even if lensing is model dependent, that the L − σ relation is valid for low luminosity high-z objects. Ad hoc observations of lensed star-forming systems are required to determine the lensing amplification accurately.

#### Constraining the dark energy equation of state with HII galaxies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
R. Chávez, M. Plionis, S. Basilakos, R. Terlevich, E. Terlevich, J. Melnick, F. Bresolin, A. L. González-Morán
Monthly Notices of the Royal Astronomical Society, Volume 462, Issue 3, 01 November 2016, Pages 2431–2439
Publication year: 2016

## Summary

We use the H II galaxies L–σ relation and the resulting Hubble expansion cosmological probe of a sample of just 25 high-z (up to z ∼ 2.33) H II galaxies, in a joint likelihood analysis with other well tested cosmological probes (cosmic microwave background, CMB, Baryon Acoustic Oscillations, BAOs) in an attempt to constrain the dark energy equation of state (EoS). The constraints, although still weak, are in excellent agreement with those of a similar joint analysis using the well established SNIa Hubble expansion probe. Interestingly, even with the current small number of available high redshift H II galaxies, the H II/BAO/CMB joint analysis gives a 13 per cent improvement of the quintessence dark energy cosmological constraints compared to the BAO/CMB joint analysis. We have further performed extensive Monte Carlo simulations, with a realistic redshift sampling, to explore the extent to which the use of the L–σ relation, observed in H II galaxies, can constrain effectively the parameter space of the dark energy EoS. The simulations predict substantial improvement in the constraints when increasing the sample of high-z H II galaxies to 500, a goal that can be achieved in reasonable observing times with existing large telescopes and state-of-the-art instrumentation.

#### Comparison of spatial and angular clustering of X-ray AGN

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
L. Koutoulidis, M. Plionis, I. Georgantopoulos, A. Georgakakis, A. Akylas, S. Basilakos and G. Mountrichas
A&A Volume 590, June 2016
Publication year: 2016

## Summary

The angular correlation function is a powerful tool for deriving the clustering properties of active galactic nuclei (AGN) and hence the mass of the corresponding dark matter halos in which they reside. However, recent studies based on the application of the angular correlation function on X-ray samples, yield results that are apparently inconsistent with those based on the direct estimation of the spatial correlation function. The goal of the present paper is to attempt to investigate this issue by analysing a well-defined sample. To this end we use the hard-band (2–10 keV) X-ray selected sources of the Chandra AEGIS fields, chosen because of the availability of accurately derived flux sensitivity maps. In particular we use the 186 hard-band sources with spectroscopic redshifts in the range z = 0.3–1.3, a range selected in order to contain the bulk of the AGN while minimizing the contribution of unknown clustering and luminosity evolution from very high redshifts. Using the projected spatial auto-correlation function, we derive a comoving clustering length of x0 = 5.4 ± 1.0h-1 Mpc (for γ = 1.8), which is consistent with results in the literature. We further derive the angular correlation function and corresponding spatial clustering length using the Limber’s inversion equation and a novel parametrization of the clustering evolution model that also takes the bias evolution of the host dark matter halo into account. The Limber’s inverted spatial comoving clustering length of x0 = 5.5 ± 1.2h-1 Mpc at a median redshift of z ≃ 0.75 matches the clustering length that is directly measured from the spatial correlation function analysis, but after introducing a significant non-linear contribution to the growing mode of perturbations; this contribution is estimated independently from literature results of x0 at different redshifts. Therefore, using this sample of hard X-ray AGN and our clustering evolution parametrization, we find an excellent consistency between the angular and spatial clustering analysis.

#### On the road to precision cosmology with high-redshift HII galaxies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
R. Chávez, R. Terlevich, E. Terlevich, F. Bresolin, J. Melnick, M. Plionis, S. Basilakos
Monthly Notices of the Royal Astronomical Society, Volume 442, Issue 4, 21 August 2014, Pages 3565–3597
Publication year: 2015

## Summary

The validity of the emission-line luminosity versus ionized gas velocity dispersion (L–σ) correlation for H II galaxies (HIIGx) and its potential as an accurate distance estimator are assessed. For a sample of 128 local (0.02 ≲ z ≲ 0.2) compact HIIGx with high equivalent widths of their Balmer emission lines, we obtained the ionized gas velocity dispersion from high signal-to-noise ratio (S/N) high-dispersion spectroscopy (Subaru High Dispersion Spectrograph (HDS) and European Southern Observatory (ESO) Very Large Telescope Ultraviolet and Visual Echelle Spectrograph (VLT–UVES)) and integrated Hβ fluxes from low-dispersion wide aperture spectrophotometry. We find that the L(Hβ)–σ relation is strong and stable against restrictions in the sample (mostly based on the emission-line profiles). The ‘Gaussianity’ of the profile is important for reducing the root-mean-square (rms) uncertainty of the distance indicator, but at the expense of substantially reducing the sample. By fitting other physical parameters into the correlation, we are able to decrease the scatter significantly without reducing the sample. The size of the star-forming region is an important second parameter, while adding the emission-line equivalent width or the continuum colour and metallicity produces the solution with the smallest rms scatter=δlog L(Hβ) = 0.233. The derived coefficients in the best L(Hβ)–σ relation are very close to what is expected from virialized ionizing clusters, while the derived sum of the stellar and ionized gas masses is similar to the dynamical mass estimated using the Hubble Space Telescope (HST) corrected Petrosian radius. These results are compatible with gravity being the main mechanism causing the broadening of the emission lines in these very young and massive clusters. The derived masses range from about 2 × 106 M to 109 M and their ‘corrected’ Petrosian radius ranges from a few tens to a few hundred pc.

#### X-ray AGN in the XMM-LSS galaxy clusters: no evidence of AGN suppression

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
E. Koulouridis, M. Plionis, O. Melnyk, A. Elyiv, I. Georgantopoulos, N. Clerc, J. Surdej, L. Chiappetti and M. Pierre
A&A Volume 567, July 2014
Publication year: 2014

## Summary

We present a study of the overdensity of X-ray-selected active galactic nuclei (AGN) in 33 galaxy clusters in the XMM-LSS field (The XMM-Newton Large Scale Structure Survey), up to redshift z = 1.05 and further divided into a lower (0.14 ≤ z ≤ 0.35) and a higher redshift (0.43 ≤ z ≤ 1.05) subsample. Previous studies have shown that the presence of X-ray-selected AGN in rich galaxy clusters is suppressed, since their number is significantly lower than what is expected from the high galaxy overdensities in the area. In the current study we have investigated the occurrence of X-ray-selected AGN in low (⟨ Lx,bol ⟩ = 2.7 × 1043 erg/s) and moderate (⟨ Lx,bol ⟩ = 2.4 × 1044 erg/s) X-ray luminosity galaxy clusters in an attempt to trace back the relation between high-density environments and nuclear activity. Owing to the wide contiguous XMM-LSS survey area, we were able to extend the study to the cluster outskirts. We therefore determined the projected overdensity of X-ray point-like sources around each cluster out to 6r500 radius, within δr500 = 1 annulus, with respect to the field expectations based on the X-ray source log N − log S of the XMM-LSS field. To provide robust statistical results we also conducted a consistent stacking analysis separately for the two z ranges. We investigated whether the observed X-ray overdensities are to be expected thanks to the obvious enhancement of galaxy numbers in the cluster environment by also estimating the corresponding optical galaxy overdensities, and we assessed the possible enhancement or suppression of AGN activity in clusters. We find a positive X-ray projected overdensity in both redshift ranges at the first radial bins, which however has the same amplitude as that of optical galaxies. Therefore, no suppression (or enhancement) of X-ray AGN activity with respect to the field is found, in sharp contrast to previous results based on rich galaxy clusters, implying that the mechanisms responsible for the suppression are not as effective in lower density environments. After a drop to roughly the background level between 2 and 3r500, the X-ray overdensity exhibits a rise at larger radii, significantly greater than the corresponding optical overdensity. The radial distance of this overdensity “bump”, corresponding to ~1.53 Mpc, depends on the richness of the clusters, as well as on the overall X-ray overdensity profile. Finally, using the redshift information, photometric or spectroscopic, of the optical counterparts, we derive the spatial overdensity profile of the clusters. We find that the agreement between X-ray and optical overdensities in the first radial bins is also suggested in the 3-dimensional analysis. However, we argue that the X-ray overdensity “bump” at larger radial distance is at least partially a result of flux boosting by gravitational lensing of background quasi-stellar objects, confirming previous results. For high-redshift clusters, the enhancement of X-ray AGN activity in their outskirts appears to be intrinsic. We argue that a spatial analysis is crucial for disentangling irrelevant phenomena affecting the projected analysis, but we are still not able to report statistically significant results on the spatial overdensity of AGN in clusters or their outskirts because we lack the necessary numbers.

#### The L–σ relation for massive bursts of star formation

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
R. Chávez, R. Terlevich, E. Terlevich, F. Bresolin, J. Melnick, M. Plionis, S. Basilakos
Monthly Notices of the Royal Astronomical Society, Volume 442, Issue 4, 21 August 2014, Pages 3565–3597
Publication year: 2014

## Summary

The validity of the emission-line luminosity versus ionized gas velocity dispersion (L–σ) correlation for H II galaxies (HIIGx) and its potential as an accurate distance estimator are assessed. For a sample of 128 local (0.02 ≲ z ≲ 0.2) compact HIIGx with high equivalent widths of their Balmer emission lines, we obtained the ionized gas velocity dispersion from high signal-to-noise ratio (S/N) high-dispersion spectroscopy (Subaru High Dispersion Spectrograph (HDS) and European Southern Observatory (ESO) Very Large Telescope Ultraviolet and Visual Echelle Spectrograph (VLT–UVES)) and integrated Hβ fluxes from low-dispersion wide aperture spectrophotometry. We find that the L(Hβ)–σ relation is strong and stable against restrictions in the sample (mostly based on the emission-line profiles). The ‘Gaussianity’ of the profile is important for reducing the root-mean-square (rms) uncertainty of the distance indicator, but at the expense of substantially reducing the sample. By fitting other physical parameters into the correlation, we are able to decrease the scatter significantly without reducing the sample. The size of the star-forming region is an important second parameter, while adding the emission-line equivalent width or the continuum colour and metallicity produces the solution with the smallest rms scatter=δlog L(Hβ) = 0.233. The derived coefficients in the best L(Hβ)–σ relation are very close to what is expected from virialized ionizing clusters, while the derived sum of the stellar and ionized gas masses is similar to the dynamical mass estimated using the Hubble Space Telescope (HST) corrected Petrosian radius. These results are compatible with gravity being the main mechanism causing the broadening of the emission lines in these very young and massive clusters. The derived masses range from about 2 × 106 M to 109 M and their ‘corrected’ Petrosian radius ranges from a few tens to a few hundred pc.

#### Precision growth index using the clustering of cosmic structures and growth data

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Athina Pouri, Spyros Basilakos, and Manolis Plionis
Journal of Cosmology and Astroparticle Physics, Volume 2014, August 2014
Publication year: 2014

## Summary

We use the clustering properties of Luminous Red Galaxies (LRGs) and the growth rate data provided by the various galaxy surveys in order to constrain the growth index γ) of the linear matter fluctuations. We perform a standard χ2-minimization procedure between theoretical expectations and data, followed by a joint likelihood analysis and we find a value of γ=0.56± 0.05, perfectly consistent with the expectations of the ΛCDM model, and Ωm0 =0.29± 0.01, in very good agreement with the latest Planck results. Our analysis provides significantly more stringent growth index constraints with respect to previous studies, as indicated by the fact that the corresponding uncertainty is only ~ 0.09 γ. Finally, allowing γ to vary with redshift in two manners (Taylor expansion around z=0, and Taylor expansion around the scale factor), we find that the combined statistical analysis between our clustering and literature growth data alleviates the degeneracy and obtain more stringent constraints with respect to other recent studies.

#### The environment of HII galaxies revisited

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
E. Koulouridis, M. Plionis, R. Chávez, E. Terlevich, R. Terlevich, F. Bresolin and S. Basilakos
A&A Volume 554, June 2013
Publication year: 2013

## Summary

We present a study of the close (≲ 200 h75-1 kpc) environment of 110 relatively local (z ≲ 0.16) HII galaxies, selected from the Sloan Digital Sky Survey (SDSS; DR7). We use available spectroscopic and photometric redshifts to investigate the presence of a close and possibly interacting companion galaxy. Our aim is to compare the physical properties of isolated and interacting HII galaxies and investigate possible systematic effects in their use as cosmological probes. We find that interacting HII galaxies tend to be more compact, less luminous and have a lower velocity dispersion than isolated ones, in agreement with previous studies on smaller samples. However, as we verify, these environmental differences do not affect the cosmologically important LHβ − σ correlation of the HII galaxies.

#### Clustering, bias and the accretion mode of X-ray-selected AGN

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
L. Koutoulidis, M. Plionis, I. Georgantopoulos, N. Fanidakis
Monthly Notices of the Royal Astronomical Society, Volume 428, Issue 2, 11 January 2013, Pages 1382–1394
Publication year: 2013

## Summary

We present the spatial clustering properties of 1466 X-ray-selected active galactic nuclei (AGN) compiled from the Chandra Deep Field (CDF) North and South, extended CDF-S, COSMOS and All-wavelength Extended Groth strip International Survey (AEGIS) fields in the 0.5–8 keV band. The X-ray sources span the redshift interval 0 < z < 3 and have a median value of z¯=0.976z¯=0.976. We employ the projected two-point correlation function to infer the spatial clustering and find a clustering length of r0 = 7.2 ± 0.6 h−1 Mpc and a slope of γ = 1.48 ± 0.12, which corresponds to a bias of b(z¯)=2.26±0.16b(z¯)=2.26±0.16. Using two different halo bias models, we consistently estimate an average dark-matter host halo mass of Mh ≃ 1.3(± 0.3) × 1013h−1 M. The X-ray AGN bias and the corresponding dark-matter host halo mass are significantly higher than the corresponding values of optically selected AGN (at the same redshifts). The redshift evolution of the X-ray-selected AGN bias indicates, in agreement with other recent studies, that a unique dark-matter halo mass does not fit well the bias at all the different redshifts probed. Furthermore, we investigate if there is a dependence of the clustering strength on X-ray luminosity. To this end we consider only 650 sources around z ∼ 1 and we apply a procedure to disentangle the dependence of clustering on redshift. We find indications for a positive dependence of the clustering length on X-ray luminosity, in the sense that the more luminous sources have a larger clustering length and hence a higher dark-matter halo mass. In detail we find for an average luminosity difference of δ log10Lx ≃ 1 a halo mass difference of a factor of ∼3.

These findings appear to be consistent with a galaxy formation model where the gas accreted on to the supermassive black hole in intermediate-luminosity AGN comes mostly from the hot-halo atmosphere around the host galaxy.

#### Classification and environmental properties of X-ray selected point-like sources in the XMM-LSS field⋆

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
O. Melnyk, M. Plionis, A. Elyiv, M. Salvato, L. Chiappetti, N. Clerc, P. Gandhi, M. Pierre, T. Sadibekova, A. Pospieszalska-Surdej and J. Surdej
A&A Volume 557, September 2013
Publication year: 2013

## Summary

Context. The XMM-Large Scale Structure survey, covering an area of 11.1 sq. deg., contains more than 6000 X-ray point-like sources detected with the XMM-Newton to a flux of 3 × 10-15 erg s-1 cm-2 in the [0.5−2] keV band. The vast majority of these sources have optical (CFHTLS), infrared (SWIRE IRAC and MIPS), near-infrared (UKIDSS), and/or ultraviolet (GALEX) counterparts.

Aims. We wish to investigate the environmental properties of the different types of the XMM-LSS X-ray sources by defining their environment using the i′-band CFHTLS W1 catalog of optical galaxies to a magnitude limit of 23.5 mag.

Methods. We have classified 4435 X-ray selected sources on the basis of their spectra, SEDs, and X-ray luminosity, and estimated their photometric redshifts, which have a 4−11 band photometry with an accuracy of σz/(1+zsp) = 0.076 with 22.6% outliers for i′ < 26 mag. We estimated the local overdensities of 777 X-ray sources that have spectro-z or photo-z calculated by using more than seven bands (accuracy of σz/(1+zsp) = 0.061 with 13.8% outliers) within the volume-limited region defined by 0.1 ≤ z ≤ 0.85 and −23.5 < Mi < −20.

Results. Although X-ray sources may be found in variety of environments, a high fraction (≳55−60%), as verified by comparing with the random expectations, reside in overdense regions. The galaxy overdensities within which X-ray sources reside show a positive recent redshift evolution (at least for the range studied; z ≲ 0.85). We also find that X-ray selected galaxies, when compared to AGN, inhabit significantly higher galaxy overdensities, although their spatial extent appear to be smaller than that of AGN. Hard AGN (HR ≥ −0.2) are located in more overdense regions than soft AGN (HR < −0.2), which is clearly seen in both redshift ranges, although it appears to be stronger in the higher redshift range (0.55 < z < 0.85). Furthermore, the galaxy overdensities (with δ ≳ 1.5) within which soft AGN are embedded appear to evolve more rapidly compared to the corresponding overdensities around hard AGN.

#### Activity of the Seyfert galaxy neighbours⋆,⋆⋆

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
E. Koulouridis, M. Plionis, V. Chavushyan, D. Dultzin, Y. Krongold, I. Georgantopoulos, and J. León-Tavares
A&A Volume 552, April 2013
Publication year: 2013

## Summary

We present a follow-up study of a series of papers concerning the role of close interactions as a possible triggering mechanism of AGN activity. We have already studied the close (≤100 h-1 kpc) and the large-scale (≤1 h-1 Mpc) environment of a local sample of Sy1, Sy2, and bright IRAS galaxies (BIRG) and of their respective control samples. The results led us to the conclusion that a close encounter appears capable of activating a sequence where an absorption line galaxy (ALG) galaxy first becomes a starburst, then a Sy2, and finally a Sy1. Here we investigate the activity of neighbouring galaxies of different types of AGN, since both galaxies of an interacting pair should be affected. To this end we present the optical spectroscopy and X-ray imaging of 30 neighbouring galaxies around two local (z ≲ 0.034) samples of 10 Sy1 and 13 Sy2 galaxies. Although this is a pilot study of a small sample, various interesting trends have been discovered that imply physical mechanisms that may lead to different Seyfert types. Based on the optical spectroscopy, we find that more than 70% of all neighbouring galaxies exhibit star forming and/or nuclear activity (namely recent star formation and/or AGN), while an additional X-ray analysis showed that this percentage might be significantly higher. Furthermore, we find a statistically significant correlation, at a 99.9% level, between the value of the neighbour’s [OIII]/Hβ ratio and the activity type of the central active galaxy, i.e. the neighbours of Sy2 galaxies are systematically more ionized than the neighbours of Sy1s. This result, in combination with trends found using the Equivalent Width of the Hα emission line and the stellar population synthesis code STARLIGHT, indicate differences in the stellar mass, metallicity, and star formation history between the samples. Our results point towards a link between close galaxy interactions and activity and also provide more clues regarding the possible evolutionary sequence inferred by our previous studies.

#### Testing general relativity using the evolution of linear bias

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, J. B. Dent, S. Dutta, L. Perivolaropoulos, and M. Plionis
PHYSICAL REVIEW D 85, 123501 (2012)
Publication year: 2012

## Summary

We investigate the cosmic evolution of the linear bias in the framework of a flat Friedmann-Lemaître-Robertson-Walker spacetime. We consider metric perturbations in the Newtonian gauge, including Hubble scale effects. Making the following assumptions, (i) scale-independent current epoch bias b0, (ii) equal accelerations between tracers and matter, (iii) unimportant halo merging effects (which is quite accurate for z<3), we analytically derive the scale-dependent bias evolution. The identified scale dependence is only due to Hubble scale evolution general relativity effects, while other scale dependence contributions are ignored. We find that up to galaxy cluster scales the fluctuations of the metric do not introduce a significant scale dependence in the linear bias. Our bias evolution model is then used to derive a connection between the matter growth index γ and the observable value of the tracer power spectrum normalization σ8(z). We show how this connection can be used as an observational test of general relativity on extragalactic scales.

#### Dynamics and constraints of the dissipative Liouville cosmology

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Spyros Basilakos, Nick E. Mavromatos, Vasiliki A. Mitsou, Manolis Plionis
Astroparticle Physics 36 (2012) 7–17
Publication year: 2012

## Summary

In this article we investigate the properties of the FLRW flat cosmological models in which the cosmic expansion of the Universe is affected by a dilaton dark energy (Liouville scenario). In particular, we perform a detailed study of these models in the light of the latest cosmological data, which serves to illustrate the phenomenological viability of the new dark energy paradigm as a serious alternative to the traditional scalar field approaches. By performing a joint likelihood analysis of the recent supernovae type Ia data (SNIa), the differential ages of passively evolving galaxies, and the baryonic acoustic oscillations (BAOs) traced by the Sloan Digital Sky Survey (SDSS), we put tight constraints on the main cosmological parameters. Furthermore, we study the linear matter fluctuation field of the above Liouville cosmological models. In this framework, we compare the observed growth rate of clustering measured from the optical galaxies with those predicted by the current Liouville models. Performing various statistical tests we show that the Liouville cosmological model provides growth rates that match well with the observed growth rate. To further test the viability of the models under study, we use the Press–Schechter formalism to derive their expected redshift distribution of cluster-size halos that will be provided by future X-ray and Sunyaev–Zeldovich cluster surveys. We find that the Hubble flow differences between the Liouville and the LambdaCDM models provide a significantly different halo redshift distribution, suggesting that the models can be observationally distinguished.

#### Determining the Hubble constant using giant extragalactic H II regions and H II galaxies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Ricardo Chávez, Elena Terlevich, Roberto Terlevich, Manolis Plionis, Fabio Bresolin, Spyros Basilakos, Jorge Melnick
Monthly Notices of the Royal Astronomical Society: Letters, Volume 425, Issue 1, September 2012, Pages L56–L60
Publication year: 2012

## Summary

We report the first results of a long-term programme aiming to provide accurate independent estimates of the Hubble constant (H0) using the L(Hβ)-σ distance estimator for giant extragalactic H II regions (GEHR) and H II galaxies.

We have used Very Large Telescope and Subaru high-dispersion spectroscopic observations of a local sample of H II galaxies, identified in the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) catalogue in order to redefine and improve the L(Hβ)-σ distance indicator and to determine the Hubble constant. To this end, we utilized as local calibration or ‘anchor’ of this correlation GEHR in nearby galaxies which have accurate distance measurements determined via primary indicators. Using our best sample of 69 nearby H II galaxies and 23 GEHR in nine galaxies, we obtain H0 = 74.3 ± 3.1 (statistical) ± 2.9 (systematic) km s−1 Mpc−1, in excellent agreement with, and independently confirming, the most recent Type Ia supernovae based results.

#### Angular correlation functions of X-ray point-like sources in the full exposure XMM-LSS field⋆

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
A. Elyiv, N. Clerc, M. Plionis, J. Surdej, M. Pierre, S. Basilakos, L. Chiappetti, P. Gandhi, E. Gosset, O. Melnyk, and F. Pacaud
A&A 537, A131 (2012)
Publication year: 2012

## Summary

Aims. Our aim is to study the large-scale structure of different types of AGN using the medium-deep XMM-LSS survey.

Methods. We measure the two-point angular correlation function of  ~5700 and 2500 X-ray point-like sources over the  ~11 sq. deg. XMM-LSS field in the soft (0.5–2 keV) and hard (2–10 keV) bands. For the conversion from the angular to the spatial correlation function we used the Limber integral equation and the luminosity-dependent density evolution model of the AGN X-ray luminosity function.

Results. We have found significant angular correlations with the power-law parameters γ = 1.81   ±   0.02, θ0 = 1.3′′    ±    0.2′′ for the soft, and γ = 2.00    ±    0.04, θ0 = 7.3′′   ±    1.0′′ for the hard bands. The amplitude of the correlation function w(θ) is higher in the hard than in the soft band for fx ≲ 10-14 erg s-1 cm-2 and lower above this flux limit. We confirm that the clustering strength θ0 grows with the flux limit of the sample, a trend which is also present in the amplitude of the spatial correlation function, but only for the soft band. In the hard band, it remains almost constant with r0 ≃ 10h-1 Mpc, irrespective of the flux limit. Our analysis of AGN subsamples with different hardness ratios shows that the sources with a hard-spectrum are more clustered than soft-spectrum ones. This result may be a hint that the two main types of AGN populate different environments. Finally, we find that our clustering results correspond to an X-ray selected AGN bias factor of  ~2.5 for the soft band sources (at a median  ≃ 1.1) and  ~3.3 for the hard band sources (at a median  ≃ 1), which translates into a host dark matter halo mass of  ~1013h-1M and  ~1013.7h-1M for the soft and hard bands, respectively.

#### Hubble expansion and structure formation in the `running FLRW model'' of the cosmic evolution

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Javier Grande, Joan Solà, Spyros Basilakos and Manolis Plionis
Journal of Cosmology and Astroparticle Physics, Volume 2011, August 2011
Publication year: 2011

## Summary

A new class of Friedmann-Lemaître-Robertson-Walker (FLRW) cosmological models with time-evolving fundamental parameters should emerge naturally from a description of the expansion of the universe based on the first principles of quantum field theory and string theory. Within this general paradigm, one expects that both the gravitational Newton’s coupling G and the cosmological term Λ should not be strictly constant but appear rather as smooth functions of the Hubble rate H(t). This scenario (“running FLRW model”) predicts, in a natural way, the existence of dynamical dark energy without invoking the participation of extraneous scalar fields. In this paper, we perform a detailed study of some of these models in the light of the latest cosmological data, which serves to illustrate the phenomenological viability of the new dark energy paradigm as a serious alternative to the traditional scalar field approaches. By performing a joint likelihood analysis of the recent supernovae type Ia data (SNIa), the Cosmic Microwave Background (CMB) shift parameter, and the Baryonic Acoustic Oscillations (BAOs) traced by the Sloan Digital Sky Survey (SDSS), we put tight constraints on the main cosmological parameters. Furthermore, we derive the theoretically predicted dark-matter halo mass function and the corresponding redshift distribution of cluster-size halos for the “running” models studied. Despite the fact that these models closely reproduce the standard ΛCDM Hubble expansion, their normalization of the perturbation’s power-spectrum varies, imposing, in many cases, a significantly different cluster-size halo redshift distribution. This fact indicates that it should be relatively easy to distinguish between the “running” models and the ΛCDM using realistic future X-ray and Sunyaev-Zeldovich cluster surveys.

#### Generalized evolution of linear bias: A tool to test gravity

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, M. Plionis, and A. Pouri
PHYSICAL REVIEW D 83, 123525 (2011)
Publication year: 2011

## Summary

We derive an exact analytical solution for the redshift evolution of linear and scale-independent bias, by solving a second-order differential equation based on linear perturbation theory. This bias evolution model is applicable to all different types of dark energy and modified gravity models. We propose that the combination of the current bias evolution model with data on the bias of extragalactic mass tracers could provide an efficient way to discriminate between geometrical dark energy models and dark energy models that adhere to general relativity.

#### Dynamics and constraints of the massive graviton dark matter flat cosmologies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, M. Plionis, M. E. S. Alves, and J. A. S. Lima
PHYSICAL REVIEW D 83, 103506 (2011)
Publication year: 2011

## Summary

We discuss the dynamics of the Universe within the framework of the massive graviton cold dark matter scenario (MGCDM) in which gravitons are geometrically treated as massive particles. In this modified gravity theory, the main effect of the gravitons is to alter the density evolution of the cold dark matter component in such a way that the Universe evolves to an accelerating expanding regime, as presently observed. Tight constraints on the main cosmological parameters of the MGCDM model are derived by performing a joint likelihood analysis involving the recent supernovae type Ia data, the cosmic microwave background shift parameter, and the baryonic acoustic oscillations as traced by the Sloan Digital Sky Survey red luminous galaxies. The linear evolution of small density fluctuations is also analyzed in detail. It is found that the growth factor of the MGCDM model is slightly different (14%) from the one provided by the conventional flat ΛCDM cosmology. The growth rate of clustering predicted by MGCDM and ΛCDM models are confronted to the observations and the corresponding best fit values of the growth index (γ) are also determined. By using the expectations of realistic future x-ray and Sunyaev-Zeldovich cluster surveys we derive the dark matter halo mass function and the corresponding redshift distribution of cluster-size halos for the MGCDM model. Finally, we also show that the Hubble flow differences between the MGCDM and the ΛCDM models provide a halo redshift distribution departing significantly from the those predicted by other dark energy models. These results suggest that the MGCDM model can observationally be distinguished from ΛCDM and also from a large number of dark energy models recently proposed in the literature.

#### A strategy to measure the dark energy equation of state using the H II galaxy Hubble function and X-ray active galactic nuclei clustering: preliminary results

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
M. Plionis, R. Terlevich, S. Basilakos, F. Bresolin, E. Terlevich, J. Melnick, R. Chavez
Monthly Notices of the Royal Astronomical Society, Volume 416, Issue 4, October 2011, Pages 2981–2996
Publication year: 2011

## Summary

We explore the possibility of setting stringent constraints to the dark energy equation of state using alternative cosmic tracers like (a) the Hubble relation using H II galaxies, which can be observed at much higher redshifts (z≲ 3.5) than those currently traced by Type Ia supernovae (SNeIa) samples, and (b) the large-scale structure using the clustering of X-ray selected active galactic nuclei (AGN), which have a redshift distribution peaking at z∼ 1.

In this paper we use extensive Monte Carlo simulations to define the optimal strategy for the recovery of the dark energy equation of state using the high-redshift (z≳ 2) Hubble relation, but accounting also for the effects of gravitational lensing, which for such high redshifts can significantly affect the derived cosmological constraints. We investigate the size of the sample of high-z H II galaxies needed to provide useful constraints in the dark energy equation of state. Based on a ‘figure of merit’ analysis, we provide estimates for the number of 2 ≲z≲ 3.5 tracers needed to reduce the cosmological solution space, presently provided by the Constitution SNIa set, by a desired factor. The analysis is given for any level of rms distance modulus uncertainty and we find that an expected reduction (i.e. by ∼20–40 per cent) of the current level of H II-galaxy-based distance modulus uncertainty does not provide a significant improvement in the derived cosmological constraints. It is much more efficient to increase the number of tracers than to reduce their individual uncertainties.

Finally, we propose a framework to put constraints on the dark energy equation of state by using the joint likelihood of the X-ray AGN clustering and of the Hubble relation cosmological analyses. A preliminary joint analysis using the X-ray AGN clustering of the 2XMM survey and the Hubble relation of the Constitution SNIa set provide Ωm= 0.31 ± 0.01 and w=−1.06 ± 0.05. We also find that the joint SNIa–2XMM analysis provides significantly more stringent cosmological constraints, increasing the figure of merit by a factor of ∼2, with respect to that of the joint SNIa–baryonic acoustic oscillation analysis.

#### The relation between halo shape, velocity dispersion and formation time

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
C. Ragone-Figueroa, M. Plionis, M. Merchán, S. Gottlöber, G. Yepes
Monthly Notices of the Royal Astronomical Society, Volume 407, Issue 1, September 2010, Pages 581–589
Publication year: 2010

## Summary

We use dark matter haloes identified in the MARENOSTRUM UNIVERSEx and galaxy groups identified in the Sloan Digital Sky Survey Data Release 7 (SDSS DR7) galaxy catalogue to study the relation between halo shape and halo dynamics, parametrizing the mass of the systems. A strong shape–dynamics correlation, independent of mass, is present in the simulation data, which we find to be due to different halo-formation times. Early formation-time haloes are, at the present epoch, more spherical and have higher velocity dispersions than late formation-time haloes. The halo shape–dynamics correlation survives the projection into two dimensions (i.e. between projected shape and 1D velocity dispersion), albeit weaker. A similar shape–dynamics correlation, independent of mass, is also found in the SDSS DR7 groups of galaxies, and in order to investigate its cause we have tested and used, as a proxy of the group formation time, a concentration parameter. We have found, as in the case of the simulated haloes, that less concentrated groups, corresponding to late formation times, have lower velocity dispersions and higher elongations than groups with higher values of concentration, corresponding to early formation times.

#### Spherical collapse model in time varying vacuum cosmologies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Spyros Basilakos, Manolis Plionis, and Joan Solà
PHYSICAL REVIEW D 82, 083512 (2010)
Publication year: 2010

## Summary

We investigate the virialization of cosmic structures in the framework of flat Friedmann-Lemaitre-Robertson-Walker cosmological models, in which the vacuum energy density evolves with time. In particular, our analysis focuses on the study of spherical matter perturbations, as they decouple from the background expansion, “turn around,” and finally collapse. We generalize the spherical collapse model in the case when the vacuum energy is a running function of the Hubble rate, Λ=Λ(H). A particularly well-motivated model of this type is the so-called quantum field vacuum, in which Λ(H) is a quadratic function, Λ(H)=n0+n2H2, with n00. This model was previously studied by our team using the latest high quality cosmological data to constrain its free parameters, as well as the predicted cluster formation rate. It turns out that the corresponding Hubble expansion history resembles that of the traditional ΛCDM cosmology. We use this Λ(t)CDM framework to illustrate the fact that the properties of the spherical collapse model (virial density, collapse factor, etc.) depend on the choice of the considered vacuum energy (homogeneous or clustered). In particular, if the distribution of the vacuum energy is clustered, then, under specific conditions, we can produce more concentrated structures with respect to the homogeneous vacuum energy case.

#### LUMINOUS X-RAY ACTIVE GALACTIC NUCLEI IN CLUSTERS OF GALAXIES

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
E. Koulouridis, and M. Plionis
The Astrophysical Journal Letters, 714:L181–L184, 2010 May 10
Publication year: 2010

## Summary

We present a study of X-ray active galactic nucleus (AGN) overdensities in 16 Abell clusters, within the redshift range 0.073 < z < 0.279, in order to investigate the effect of the hot inter-cluster environment on the triggering of the AGN phenomenon. The X-ray AGN overdensities, with respect to the field expectations, were estimated for sources with Lx ≥ 1042 erg s–1 (at the redshift of the clusters) and within an area of 1 h –1 72 Mpc radius (excluding the core). To investigate the presence or absence of a true enhancement of luminous X-ray AGNs in the cluster area, we also derived the corresponding optical galaxy overdensities, using a suitable range of r-band magnitudes. We always find the latter to be significantly higher (and only in two cases roughly equal) with respect to the corresponding X-ray overdensities. Over the whole cluster sample, the mean X-ray point-source overdensity is a factor of ~4 less than that corresponding to bright optical galaxies, a difference which is significant at a >0.995 level, as indicated by an appropriate student’s t-test. We conclude that the triggering of luminous X-ray AGNs in rich clusters is strongly suppressed. Furthermore, searching for optical Sloan Digital Sky Survey counterparts of all the X-ray sources, associated with our clusters, we found that about half appear to be background QSOs, while others are background and foreground AGNs or stars. The true overdensity of X-ray point sources, associated with the clusters, is therefore even smaller than what our statistical approach revealed.

#### Confronting dark energy models using galaxy cluster number counts

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, M. Plionis, and J. A. S. Lima
PHYSICAL REVIEW D 82, 083517 (2010)
Publication year: 2010

## Summary

The mass function of cluster-size halos and their redshift distribution are computed for 12 distinct accelerating cosmological scenarios and confronted to the predictions of the conventional flat ΛCDM model. The comparison with ΛCDM is performed by a two-step process. First, we determine the free parameters of all models through a joint analysis involving the latest cosmological data, using supernovae type Ia, the cosmic microwave background shift parameter, and baryon acoustic oscillations. Apart from a braneworld inspired cosmology, it is found that the derived Hubble relation of the remaining models reproduces the ΛCDM results approximately with the same degree of statistical confidence. Second, in order to attempt to distinguish the different dark energy models from the expectations of ΛCDM, we analyze the predicted cluster-size halo redshift distribution on the basis of two future cluster surveys: (i) an X-ray survey based on the eROSITA satellite, and (ii) a Sunayev-Zeldovich survey based on the South Pole Telescope. As a result, we find that the predictions of 8 out of 12 dark energy models can be clearly distinguished from the ΛCDM cosmology, while the predictions of 4 models are statistically equivalent to those of the ΛCDM model, as far as the expected cluster mass function and redshift distribution are concerned. The present analysis suggests that such a technique appears to be very competitive to independent tests probing the late time evolution of the Universe and the associated dark energy effects.

#### BREAKING THE σ8–Ω m DEGENERACY USING THE CLUSTERING OF HIGH-z X-RAY ACTIVE GALACTIC NUCLEI

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Spyros Basilakos, and Manolis Plionis
The Astrophysical Journal Letters, 714:L185–L189, 2010 May 10
Publication year: 2010

## Summary

The clustering of X-ray selected active galactic nuclei (AGNs) appears to be a valuable tool for extracting cosmological information. Using the recent high-precision angular clustering results of ~30,000 XMM-Newton soft (0.5-2 keV) X-ray sources, which have a median redshift of z ~ 1, and assuming a flat geometry, a constant in comoving coordinates AGN clustering evolution, and the AGN bias evolution model of Basilakos et al., we manage to break the σ8 m degeneracy. The resulting cosmological constraints are Ω m = 0.27+0.03 –0.05, w = –0.90+0.10 –0.16, and σ8 = 0.74+0.14 –0.12, while the dark matter host halo mass, in which the X-ray selected AGNs are presumed to reside, is M = 2.50+0.50 –1.50 × 1013 h –1 M . For the constant Λ model (w = –1) we find Ω m = 0.24 ± 0.06 and σ8 = 0.83+0.11 –0.16, in good agreement with recent studies based on cluster abundances, weak lensing, and the cosmic microwave background, but in disagreement with the recent bulk flow analysis.

#### THE RELATION BETWEEN MORPHOLOGY AND DYNAMICS OF POOR GROUPS OF GALAXIES

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Hrant M., Tovmassian, and M. Plionis
The Astrophysical Journal, 696:1441–1447, 2009 May 10
Publication year: 2009

## Summary

We investigate the relation between the projected morphology and the velocity dispersion of groups of galaxies using two recently compiled group catalogs, one based on the Two Micron All Sky Survey redshift survey (Crook et al.) and the other on the Sloan Digital Sky Survey Data Release 5 galaxy catalog (Tago et al.). We analyze a suitable subsample of groups from each catalog selected such that it minimizes possible systematic effects. We find that the velocity dispersion of groups is strongly correlated with the group-projected shape and size, with elongated and larger groups having a lower velocity dispersion. Such a correlation could be attributed to the dynamical evolution of groups, with groups in the initial stages of formation, before virialization is complete, having small velocity dispersion, a large size, and an elongated shape that reflects the anisotropic accretion of galaxies along filamentary structures. However, we show that the same sort of correlations could also be reproduced in prolatelike groups, irrespective of their dynamical state, if the net galaxy motion is preferentially along the group elongation, since then the groups oriented close to the line of sight will appear more spherical, will have a small projected size and high-velocity dispersion, while groups oriented close to the sky plane will appear larger in projection, more elongated, and will have smaller velocity dispersion. Although both factors must play a role in shaping the observed correlations, we attempt to disentangle them by performing tests that relate only to the dynamical evolution of groups (i.e., calculating the fraction of early-type galaxies in groups and the projected group compactness). Indeed we find a strong positive (negative) correlation between the group velocity dispersion (group-projected major axis) with the fraction of early-type galaxy members. We conclude that (1) the observed dependences of the group velocity dispersion on the group-projected size and shape, should be attributed mostly to the dynamical state of groups, and (2) groups of galaxies in the local universe do not constitute a family of objects in dynamical equilibrium, but rather a family of cosmic structures that are presently at various stages of their virialization process.

#### Richness dependence of the recent evolution of clusters of galaxies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis, Hrant M. Tovmassian, Heinz Andernach
Monthly Notices of the Royal Astronomical Society, Volume 395, Issue 1, May 2009, Pages 2–10
Publication year: 2009

## Summary

We revisit the issue of the recent dynamical evolution of clusters of galaxies using a sample of Abell, Corwin & Olowin (ACO) clusters with z < 0.14, which has been selected such that it does not contain clusters with multiple velocity components nor strongly merging or interacting clusters, as revealed in X-rays. We use as proxies of the cluster dynamical state the projected cluster ellipticity, velocity dispersion and X-ray luminosity. We find indications for a recent dynamical evolution of this cluster population, which however strongly depends on the cluster richness. Poor clusters appear to be undergoing their primary phase of virialization, with their ellipticity increasing with redshift with a rate dε/dz≃ 2.5 ± 0.4, while the richest clusters show an ellipticity evolution in the opposite direction (with dε/dz≃−1.2 ± 0.1), which could be due to secondary infall. When taking into account sampling effects due to the magnitude-limited nature of the ACO cluster catalogue we find no significant evolution of the cluster X-ray luminosity, while the velocity dispersion increases with decreasing redshift, independent of the cluster richness, at a rate dσv/dz≃−1700 ± 400 km s−1.

#### Precision cosmology from X-ray AGN clustering

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Spyros Basilakos, Manolis Plionis
Monthly Notices of the Royal Astronomical Society: Letters, Volume 400, Issue 1, November 2009, Pages L57–L60
Publication year: 2009

## Summary

We place tight constraints on the main cosmological parameters of spatially flat cosmological models by using the recent angular clustering results of XMMNewton soft (0.5–2 keV) X-ray sources, which have a redshift distribution with a median of z∼ 1. Performing a standard likelihood procedure, assuming a constant in comoving coordinates active galactic nuclei (AGN) clustering evolution, the AGN bias evolution model of Basilakos, Plionis & Ragone-Figueroa and the Wilkinson Microwave Anisotropy Probe5 value of σ8, we find stringent simultaneous constraints in the (Ωmw) plane, with Ωm= 0.26 ± 0.05, w=−0.93+0.11−0.19.

#### Hubble expansion and structure formation in time varying vacuum models

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Spyros Basilakos, Manolis Plionis, and Joan Solà
PHYSICAL REVIEW D 80, 083511 (2009)
Publication year: 2009

## Summary

We investigate the properties of the FLRW flat cosmological models in which the vacuum energy density evolves with time, Λ(t). Using different versions of the Λ(t) model, namely, quantum field vacuum, power series vacuum and power law vacuum, we find that the main cosmological functions such as the scale factor of the Universe, the Hubble expansion rate H, and the energy densities are defined analytically. Performing a joint likelihood analysis of the recent supernovae type Ia data, the cosmic microwave background shift parameter and the baryonic acoustic oscillations traced by the Sloan Digital Sky Survey galaxies, we put tight constraints on the main cosmological parameters of the Λ(t) scenarios. Furthermore, we study the linear matter fluctuation field of the above vacuum models. We find that the patterns of the power series vacuum Λ=n1H+n2H2 predict stronger small scale dynamics, which implies a faster growth rate of perturbations with respect to the other two vacuum cases (quantum field and power law), despite the fact that all the cosmological models share the same equation of state parameter. In the case of the quantum field vacuum Λ=n0+n2H2, the corresponding matter fluctuation field resembles that of the traditional Λ cosmology. The power law vacuum (Λan) mimics the classical quintessence cosmology, the best fit being tilted in the phantom phase. In this framework, we compare the observed growth rate of clustering measured from the optical galaxies with those predicted by the current Λ(t) models. Performing a Kolmogorov-Smirnov statistical test we show that the cosmological models which contain a constant vacuum (ΛCDM), quantum field vacuum, and power law vacuum provide growth rates that match well with the observed growth rate. However, this is not the case for the power series vacuum models (in particular, the frequently adduced ΛH model) in which clusters form at significantly earlier times (z4) with respect to all other models (z2). Finally, we derived the theoretically predicted dark matter halo mass function and the corresponding distribution of cluster-size halos for all the models studied. Their expected redshift distribution indicates that it will be difficult to distinguish the closely resembling models (constant vacuum, quantum field, and power law vacuum), using realistic future x-ray surveys of cluster abundances. However, cluster surveys based on the Sunayev-Zeldovich detection method give some hope to distinguish the closely resembling models at high redshifts.

#### Could dark matter interactions be an alternative to dark energy?

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos1 and M. Plionis2
A&A 507, 47–52 (2009)
Publication year: 2009

## Summary

We study the global dynamics of the universe within the framework of the interacting dark matter (IDM) scenario. Assuming that the dark matter obeys the collisional Boltzmann equation, we can derive analytical solutions of the global density evolution, that can accommodate an accelerated expansion, equivalent to either the quintessence or the standard Λ models, with the present time located after the inflection point. This is possible if there is a disequilibrium between the DM particle creation and annihilation processes with the former process dominating, which creates an effective source term with negative pressure. Comparing the predicted Hubble expansion of one of the IDM models (the simplest) with observational data, we find that the effective annihilation term is quite small, as suggested by various experiments.

#### The Halo Mass-Bias Redshift Evolution in the ΛCDM Cosmology

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, M. Plionis, and C. Ragone-Figueroa
The Astrophysical Journal 678 (2008) 627
Publication year: 2008

## Summary

We derive an analytic model for the redshift evolution of linear bias, allowing for interactions and merging of the mass tracers, by solving a second-order differential equation based on linear perturbation theory and the Friedmann-Lemaitre solutions of the cosmological field equations. We then study the halo-mass dependence of the bias evolution, using the dark matter halo distribution in a ΛCDM simulation in order to calibrate the free parameters of the model. Finally, we compare our theoretical predictions with available observational data and find a good agreement. In particular, we find that the bias of optical QSO’s evolve differently than those selected in X-rays and that their corresponding typical dark matter halo mass is ~1013 h−1 M and × 1013 h−1 M, respectively.

#### The Aspen–Amsterdam void finder comparison project

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Jörg M. Colberg, Frazer Pearce, Caroline Foster, Erwin Platen, Riccardo Brunino, Mark Neyrinck, Spyros Basilakos, Anthony Fairall, Hume Feldman, Stefan Gottlöber, Oliver Hahn, Fiona Hoyle, Volker Müller, Lorne Nelson, Manolis Plionis, Cristiano Porciani, Sergei Shandarin, Michael S. Vogeley, Rien Van De Weygaert
Monthly Notices of the Royal Astronomical Society, Volume 387, Issue 2, June 2008, Pages 933–944
Publication year: 2008

## Summary

Despite a history that dates back at least a quarter of a century, studies of voids in the large-scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void-finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of 13 different void finders constructed using particles, haloes, and semi-analytical model galaxies extracted from a subvolume of the Millennium simulation. This study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 per cent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than mB=−20 is found to be smaller than about −0.8 by all our void finding algorithms.

#### Luminosity-dependent X-Ray Active Galactic Nucleus Clustering?

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
M. Plionis, M. Rovilos, S. Basilakos, I. Georgantopoulos, and F. Bauer
The Astrophysical Journal 674 (2008) L5
Publication year: 2008

## Summary

We have analyzed the angular clustering of X-ray-selected active galactic nuclei (AGNs) in different flux-limited subsamples of the Chandra Deep Field North (CDF-N) and South (CDF-S) surveys. We find a strong dependence of the clustering strength on the subsample flux limit, a fact that explains most of the disparate clustering results of different XMM and Chandra surveys. Using Limber’s equation, we find that the inverted CDF-N and CDF-S spatial clustering lengths are consistent with direct spatial clustering measures found in the literature, while at higher flux limits the clustering length increases considerably; for example, at fx,limit ~ 10−15 ergs s−1 cm−2, we obtain r0 17 ± 5 and 18 ± 3 h−1 Mpc for the CDF-N and CDF-S, respectively. We show that the observed flux limit clustering trend hints toward an X-ray luminosity dependent clustering of X-ray-selected, z ~ 1, AGNs.

#### Large-scale structure in the H I Parkes All-Sky Survey: filling the voids with H I galaxies?

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, M. Plionis, K. Kovač, N. Voglis
Monthly Notices of the Royal Astronomical Society, Volume 378, Issue 1, June 2007, Pages 301–308
Publication year: 2007

## Summary

We estimate the two-point correlation function in redshift space of the recently compiled H I Parkes All-Sky Survey neutral hydrogen (H I) sources catalogue, which if modelled as a power law, ξ(r) = (r0/r)γ, the best-fitting parameters for the H I selected galaxies are found to be r0= 3.3 ± 0.3 h−1 Mpc with γ= 1.38 ± 0.24. Fixing the slope to its universal value γ= 1.8, we obtain r0= 3.2 ± 0.2 h−1 Mpc.

Comparing the measured two-point correlation function with the predictions of the concordance cosmological model (ΩΛ= 0.74), we find that at the present epoch the H I selected galaxies are antibiased with respect to the underlying matter fluctuation field with their bias value being b0≃ 0.68. Furthermore, dividing the H I galaxies into two richness subsamples we find that the low-mass H I galaxies have a very low present bias factor (b0≃ 0.48), while the high-mass H I galaxies trace the underlying matter distribution as the optical galaxies (b0≃ 1). Using our derived present-day H I galaxy bias we estimate their redshift-space distortion parameter, and correct accordingly the correlation function for peculiar motions. The resulting real-space correlation length is rre0= 1.8 ± 0.2 h−1 Mpc and rre0= 3.9 ± 0.6 h−1 Mpc for the low- and high-mass H I galaxies, respectively. The low-mass H I galaxies appear to have the lowest correlation length among all extragalactic populations studied to date. In order to corroborate these results we have correlated the IRAS–Point Source Catalogue for Redshift (PSCz) reconstructed density field, smoothed over scales of 5 h−1 Mpc, with the positions of the H I galaxies, to find that indeed the H I galaxies are typically found in negative overdensity regions (δρ/ρPSCz≲ 0), even more so the low-mass H I galaxies.

Finally, we also study the redshift evolution of the H I galaxy linear bias factor and find that the H I-galaxy population is antibiased up to z∼ 1.3. While at large redshifts z∼ 3, we predict that the H I galaxies are strongly biased. Our bias evolution predictions are consistent with the observational bias results of Lyα galaxies.

#### Environmental influences on the morphology and dynamics of group-sized haloes

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Cinthia Ragone-Figueroa, Manolis Plionis
Monthly Notices of the Royal Astronomical Society, Volume 377, Issue 4, June 2007, Pages 1785–1794
Publication year: 2007

## Summary

We use group-sized haloes, with masses in the range 1013 < M < 2 × 1014h−1 M, identified with a ‘friends of friends’ (FOF) algorithm in a concordance Λ cold dark matter (ΛCDM) GADGET2 (dark matter only) simulation to investigate the dependence of halo properties on the environment at z= 0. The study is carried out using samples of haloes at different distances from their nearest massive cluster halo, considered as such if its mass is larger than the upper limit of the above halo mass range (i.e. M≥ 2 × 1014h−1 M). We find that the fraction of haloes with substructure typically increases in high-density regions. The halo mean axial ratio 〈c/a〉 also increases in overdense regions, a fact which is true for the whole range of halo mass studied. This can be explained as a reflection of an earlier halo formation time in high-density regions, which gives haloes more time to evolve and become more spherical. Moreover, this interpretation is supported by the fact that, at a given halo–cluster distance, haloes with substructure are more elongated than their equal mass counterparts with no substructure, reflecting that the virialization (and thus sphericalization) process is interrupted by merger events. The velocity dispersion of low-mass haloes with strong substructure shows a significant increase near massive clusters with respect to equal mass haloes with low levels of substructure or with haloes found in low-density environments. The alignment signal between the shape and the velocity ellipsoid principal axes decreases going from lower to higher density regions, while such an alignment is stronger for haloes without substructure. We also find, in agreement with other studies, a tendency of halo major axes to be aligned and of minor axes to lie roughly perpendicular with the orientation of the filament within which the halo is embedded, an effect which is stronger in the proximity of the massive clusters.

#### Environmental Effects of Dark Matter Haloes: The Clustering-Substructure relation of Group-size Haloes

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
N. Espino-Briones, M. Plionis, C. Ragone-Figueroa
The Astrophysical Journal, 666:L5-L8, 2007 September 1
Publication year: 2007

## Summary

We estimate the two-point correlation function of dark matter haloes, with masses >10^{13} h^{-1} Mo, that have or not significant substructure. The haloes are identified with a friends of friends algorithm in a large LCDM simulation at two redshift snapshots (z=0 and 1), while halo substructure is determined using an observationally driven method. We find in both epochs a clear and significant signal by which haloes with substructure are more clustered than those with no-substructure. This is true for all the considered halo mass ranges, although for the highest halo masses the signal is noisy and present only out to ~20 h^{-1} Mpc. There is also a smooth increase of the halo correlation length with increasing amplitude of the halo substructure. We also find that substructured haloes are typically located in high-density large-scale environments, while the opposite is true for non-substructured haloes. If the haloes found in high-density regions have a relatively earlier formation time, as suggested by recent works, then they do indeed have more time to cluster than haloes, of a similar mass, which form later in the low-density regions. In such a case one would have naively expected that the former (earlier formed) haloes would typically be dynamically more relaxed than the latter (later formed). However, the higher merging and interaction rate,expected in high-density regions, could disrupt their relatively relaxed dynamical state and thus be the cause for the higher fraction of haloes with substructure found in such regions.

#### The shape-alignment relation in λ cold dark matter cosmic structures

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, M. Plionis, G. Yepes, S. Gottlöber, V. Turchaninov
Monthly Notices of the Royal Astronomical Society, Volume 365, Issue 2, January 2006, Pages 539–547
Publication year: 2006

## Summary

In this paper, we study the supercluster-cluster morphological properties using one of the largest (2 × 5123) smoothed particle hydrodynamics (SPH)+N-body simulations of large-scale structure formation in a λ cold dark matter (λCDM) model, based on the publicly available code GADGET. We find that filamentary (prolate-like) shapes are the dominant supercluster and cluster dark matter halo morphological feature, in agreement with previous studies. However, the baryonic gas component of the clusters is predominantly spherical. We investigate the alignment between cluster haloes (using either their dark matter or their baryonic components) and their parent supercluster major-axis orientation, finding that clusters show such a preferential alignment. Combining the shape and the alignment statistics, we also find that the amplitude of supercluster-cluster alignment increases, although weakly, with supercluster filamentariness.

#### The Serendipitous XMM-Newton Cluster Athens Survey (SEXCLAS): sample selection and the cluster log N-log S

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
V. Kolokotronis, A. Georgakakis, S. Basilakos, S. Kitsionas, M. Plionis, I. Georgantopoulos, T. Gaga
Monthly Notices of the Royal Astronomical Society, Volume 366, Issue 1, February 2006, Pages 163–170
Publication year: 2006

## Summary

In this paper we serendipitously identify X-ray cluster candidates using XMM-Newton archival observations complemented by five-band optical photometric follow-up observations (r≈ 23 mag) as part of the X-ray Identification (XID) programme. Our sample covers an area of ≈2.1 deg2 (15 XMM-Newton fields) and comprises a total of 21 (19 serendipitous + two target) extended X-ray sources to the limit ƒx (0.5-2 keV) ≈ 6 × 10−15 erg s−1 cm−2, with a high probability (>99.9 per cent) of being extended on the XMM-Newton images. Of the 21 X-ray clusters, 14 are detected for the first time while seven are spectroscopically confirmed in the literature. Exploiting the optical data available for these fields we discover that ≳68 per cent of the X-ray cluster candidates are associated with optical galaxy overdensities. We also attempt to constrain the redshifts of our cluster candidates using photometric methods. We thus construct the photometric redshift distribution of galaxies in the vicinity of each X-ray selected cluster candidate and search for statistically significant redshift peaks against that of the background distribution of field galaxies. Most of our clusters have photometric or spectroscopic redshifts in the range 0.4 < z < 0.6. Comparison of photometric with spectroscopic redshift estimates for the confirmed clusters suggests that our simple method is robust out to z≈ 0.5. For clusters at higher z, deeper optical data are required to estimate reliable photometric redshifts. Using the sample of the 19 serendipitous X-ray selected cluster candidates, we estimate their surface density down to ƒx (0.5-2 keV) ≈ 6 × 10−15 erg s−1 cm−2 and find it to be in fair agreement with previous and recent studies.

#### The PSCz dipole revisited

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Spyros Basilakos, Manolis Plionis
Monthly Notices of the Royal Astronomical Society, Volume 373, Issue 3, December 2006, Pages 1112–1116
Publication year: 2006

## Summary

We re-examine the gravitational acceleration (dipole) induced on the Local Group of galaxies by the Infrared Astronomical Satellite (IRAS) galaxy distribution of the Point Source Catalog redshift (PSCz) survey. We treat the cirrus-affected low galactic latitudes by utilizing a spherical harmonic expansion of the galaxy surface density field up to the octapole order. We find strong indications for significant contributions to the Local Group motion from depths up to ∼185 h−1 Mpc and a possible contribution even from ∼210 h−1 Mpc, in agreement with the recent analysis of Kocevski & Ebeling of a whole-sky X-ray cluster survey. What changes with respect to the previous PSCz dipole analyses is: (i) the large-scale dipole contributions and (ii) an increase of the overall dipole amplitude due to the important contribution of the local volume (≲ 4 h−1 Mpc), which we now take into account. This results in a lower value of the β (≡Ω0.6m/b) parameter, which we find to be βIRAS≃ 0.49 in real space. Therefore, for the concordance cosmological model (Ωm= 1 −ΩΛ= 0.3), the IRAS galaxies bias factor is bIRAS≃ 1, which means that IRAS galaxies are good tracers of the underlying matter distribution.

#### Physical properties of Hickson compact groups and of the loose groups within which they are embedded

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
H. Tovmassian, M. Plionis, and J. P. Torres-Papaqui
A&A 456, 839–846 (2006)
Publication year: 2006

## Summary

Using new data and an enlarged group sample we verify some of our previously published results and present a number of new facts that suggest that compact groups could be casual concentrations in prolate-like looser groups, and thus the nature of compact and ordinary poor groups is probably the same. To this end we used the Sloan Digital Sky Survey (SDSS) redshift catalogue to look for galaxies with accordant redshifts in the nearby environment (up to ~2 Mpc) of 15 Hickson compact groups (HCG). We also used known member redshifts of looser groups in the environment of 7 other HCGs. From this sample of 22 HCGs we find that: (a) HCG’s tend to be aligned with the overall galaxy distribution in their ~1 Mpc environment; (b) the well-established orientation effect by which the group velocity dispersion  correlates with group axial ratio q is present and particularly strong also in the HCG + environment systems; (c) the velocity dispersion  of the HCG + environment systems as well as of ordinary poor groups, depends only weakly on the group richness, i.e. on the mass; (d) the mean absolute K-band magnitude of E/S0 galaxies in HCGs is similar to the corresponding one in ordinary poor groups and is brighter than that of isolated E/S0’s, indicating that they were formed by the merging of two galaxies of similar luminosity; (e) the fraction of E/S0 galaxies in these HCGs depends, albeit weakly, on the group’s richness and on ; (f) the fraction of AGNs is similar in the HCGs and their close environment, while the fraction of starburst galaxies is significantly higher in the HCGs; (g) the fraction of active galaxies (AGNs and starbursts) is anti-correlated with the velocity dispersion of the HCG + environment systems. The combination of all the above facts constitutes a picture in which compact groups are condensations within looser prolate-like, elongated systems, and they appear to be compact when their member galaxies, moving in radial orbits along the group elongation, happen to come close to each other (in which case dynamical interactions among these galaxies become even more probable) or when the group is oriented close to the line of sight, so that many of its members are projected over a small solid angle. The probability of either case is small, so the number of CGs should be much smaller than that of ordinary groups, as observed. Furthermore, the observed fractions of early-type and active galaxies, as well as their correlations with the group velocity dispersion suggests a picture by which nuclear activity and galaxy transformation by merging is instigated by effective gravitational interactions in the low-velocity dispersion groups, which then dynamically evolve via virialization processes to higher velocity dispersion groups, which thus have a higher fraction of early-type galaxies.

#### Morphological and Dynamical Properties of Low-Redshift Two Degree Field Galaxy Redshift Survey Groups

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
M. Plionis, S. Basilakos, and C. Ragone-Figueroa
The Astrophysical Journal, 650:770Y776, 2006 October 20
Publication year: 2006

## Summary

We estimate the average group morphological and dynamical characteristics of the Percolation-Inferred Galaxy Group (2PIGG) catalog within z ≤ 0.08, for which the group space density is roughly constant. We quantify the different biases that enter in the determination of these characteristics, and we devise statistical correction procedures to recover their bias-free values. We find that the only acceptable morphological model is that of prolate, or triaxial with pronounced prolateness, group shapes having a roughly Gaussian intrinsic axial ratio distribution with mean ~0.46 and dispersion ~0.16. After correcting for various biases, the most important of which is a redshift-dependent bias, the median values of the virial mass and virial radius of groups with 4-30 galaxy members are v ~ 6 × 1012 h Mv ~ 0.4 h Mpc, which are significantly smaller than recent literature values that do not take into account the previously mentioned biases. The group mean crossing time is ~1.5 Gyr, independent of the group galaxy membership. We also find that there is a correlation of the group size, velocity dispersion, and virial mass with the number of group member galaxies, a manifestation of the hierarchy of cosmic structures.

#### Mining for normal galaxies in the first XMM–Newton Serendipitous Source Catalog

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
A. E. Georgakakis, V. Chavushyan, M. Plionis, I. Georgantopoulos, E. Koulouridis, I. Leonidaki, A. Mercado
Monthly Notices of the Royal Astronomical Society, Volume 367, Issue 3, April 2006, Pages 1017–1028
Publication year: 2006

## Summary

This paper uses the first XMM–Newton Serendipitous Source Catalog compiled by the XMM–Newton Science Centre to identify low-z X-ray selected normal galaxy candidates. Our sample covers a total area of ≈6 deg2 to the 0.5–2 keV limit ≈10−15 erg s−1 cm−2. A total of 23 sources are selected on the basis of low X-ray to optical flux ratio log fX/fopt < −2, soft X-ray spectral properties and optical spectra, when available, consistent with stellar formation rather than active galactic nucleus (AGN) processes. This sample is combined with similarly selected systems from the Needles in the Haystack Survey to provide a total of 46 unique (z≲ 0.2) X-ray detected normal galaxies, the largest low-z sample yet available. This is first used to constrain the normal galaxy log N–log S at bright fluxes (10−15–10−13 erg s−1 cm−2). We estimate a slope of −1.46 ± 0.13 for the cumulative number counts consistent with the Euclidean prediction. We further combine our sample with 23 local (z≲ 0.2) galaxies from the Chandra Deep Field-North and -South surveys to construct the local X-ray luminosity function of normal galaxies. A Schechter form provides a good fit to the data with a break at log L= 41.02+0.14−0.12 erg s−1 and a slope of α=−1.76 ± 0.10. Finally, for the sample of 46 systems, we explore the association between X-ray luminosity and host galaxy properties, such as star formation rate (SFR) and stellar mass. We find that the LX of the emission-line systems correlates with Hα luminosity and 1.4-GHz radio power, both providing an estimate of the current SFR. In the case of early-type galaxies with absorption-line optical spectra, we use the K band as an approximation of stellar mass and find a correlation of the form LXL1.5K. This is flatter than the LXLB relation for local ellipticals. This may be due to either LK providing a better approximation of galaxy mass or selection effects biasing our sample against very luminous early-type galaxies, LX > 1042 erg s−1.

#### Local and Large-Scale Environment of Seyfert Galaxies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
E. Koulouridis, M. Plionis, V. Chavushyan, D. Dultzin-Hacyan, Y. Krongold, and C. Goudis
The Astrophysical Journal, 639:37–45, 2006 March 1
Publication year: 2006

## Summary

We present a three-dimensional study of the local (≤100 h-1 kpc) and the large-scale (≤1 h-1 Mpc) environment of the two main types of Seyfert AGN galaxies. For this purpose we use 48 Seyfert 1 galaxies (with redshifts in the range 0.007 ≤ z ≤ 0.036) and 56 Seyfert 2 galaxies (with 0.004 ≤ z ≤ 0.020), located at high galactic latitudes, as well as two control samples of nonactive galaxies having the same morphological, redshift, and diameter size distributions as the corresponding Seyfert samples. Using the Center for Astrophysics (CfA2) and Southern Sky Redshift Survey (SSRS) galaxy catalogs (mB ~ 15.5) and our own spectroscopic observations (mB ~ 18.5), we find that within a projected distance of 100 h-1 kpc and a radial velocity separation of δv  600 km s-1 around each of our AGNs, the fraction of Seyfert 2 galaxies with a close neighbor is significantly higher than that of their control (especially within 75 h-1 kpc) and Seyfert 1 galaxy samples, confirming a previous two-dimensional analysis of Dultzin-Hacyan et al. We also find that the large-scale environment around the two types of Seyfert galaxies does not vary with respect to their control sample galaxies. However, the Seyfert 2 and control galaxy samples do differ significantly when compared to the corresponding Seyfert 1 samples. Since the main difference between these samples is their morphological type distribution, we argue that the large-scale environmental difference cannot be attributed to differences in nuclear activity but rather to their different type of host galaxies.

#### Constraining the Cold Dark Matter Spectrum Normalization in Flat Dark Energy Cosmologies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Spyros Basilakos, and Manolis Plionis
The Astrophysical Journal, 650: L1–L4, 2006 October 10
Publication year: 2006

## Summary

We study the relation between the rms mass fluctuations on 8 h-1 Mpc scales and Ωm using the recent clustering results of XMM-Newton soft (0.5-2 keV) X-ray sources, which have a median redshift of z ~ 1.2. The relation can be represented in the form σ8 = 0.34(±0.01)Ω, where γ ≡ γ(Ωmw), and it is valid for all w < –  models. By combining the X-ray clustering and SN Ia data we find that the model that best reproduces the observational data is that with Ωm  0.26, w  -0.90, and σ8  0.73, which is in excellent agreement with the recent 3 yr Wilkinson Microwave Anisotropy Probe results.

#### A Three-dimensional Study of the Local Environment of Bright IRAS Galaxies: The Active Galactic Nucleus-Starburst Connection

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Elias Koulouridis, Vahram Chavushyan, Manolis Plionis, Yair Krongold, and Deborah Dultzin-Hacyan
The Astrophysical Journal, 651:93Y100, 2006 November 1
Publication year: 2006

## Summary

We present a three-dimensional study of the local (≤100 h-1 kpc) and the large scale (≤1 h-1 Mpc) environment of bright IRAS balaxies (BIRGs). For this purpose we use 87 BIRGs located at high Galactic latitudes (with 0.008 ≤ z ≤ 0.018), as well as a control sample of nonactive galaxies having the same morphological, redshift, and diameter size distributions as the corresponding BIRG sample. Using the Center for Astrophysics and the Southern Sky Redshift Survey galaxy catalogs (mb  15.5), as well as our own spectroscopic observations (mb  19.0), for a subsample of the original BIRG sample, we find that the fraction of BIRGs with a close neighbor is significantly higher than that of their control sample. Comparing with a related analysis of Seyfert 1 and Seyfert 2 galaxies by Koulouridis and coworkers, we find that BIRGs have a similar environment to that of Seyfert 2 galaxies, although the fraction of BIRGs with a bright, close neighbor is even higher than that for Seyfert 2 galaxies. An additional analysis of the relation between FIR colors and the type of activity of each BIRG shows a significant difference between the colors of strongly interacting and noninteracting starbursts and a resemblance between the colors of noninteracting starbursts and Seyfert 2 galaxies. Our results support the view that close interactions can drive molecular clouds toward the galactic center, triggering starburst activity and obscuring the nuclear activity. When the close neighbor moves away, starburst activity is reduced with the simultaneous appearance of an obscured (type 2) active galactic nucleus (AGN). Finally, the complete disentanglement of the pair gives birth to an unobscured (type 1) AGN.

#### XMM-Newton Observations of Optically Selected Sloan Digital Sky Survey Clusters

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
M. Plionis, S. Basilakos, I. Georgantopoulos, and A. Georgakakis
The Astrophysical Journal, 622:L17–L20, March 20 2005
Publication year: 2005

## Summary

We explore the X-ray properties of a subset of the optically selected Sloan Digital Sky Survey (SDSS) cluster sample of Goto et al. by analyzing seven public XMM-Newton pointings, with exposure times ranging from ~4 to 46 ks. There are in total 17 SDSS clusters out of which only eight are detected at X-ray wavelengths with f0.5-2 keV  1.2 × 10-14 ergs cm-2 s-1. For the remaining nine SDSS clusters, we estimate their 3 σ luminosity upper limits (corresponding to LX  5 × 1042 ergs s-1 in the 0.5-2 keV band). This relatively low luminosity suggests that if they are real structures, these galaxy aggregations correspond to poor groups of galaxies. Using the SDSS photometric catalog, we also derive the cluster optical r-band luminosities. The resulting scaling relations (LoptLXLoptTX) are consistent with those of other recent studies.

#### The XMM—Newton/2dF survey — VIII. The extended X-ray sources

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
T. Gaga, M. Plionis, S. Basilakos, I. Georgantopoulos, A. Georgakakis
Monthly Notices of the Royal Astronomical Society, Volume 363, Issue 3, November 2005, Pages 811–817
Publication year: 2005

## Summary

We present a sample of eight extended X-ray sources detected in the wide-field (∼2.3 deg2), bright (2–10 ks) XMM—Newton/2dF survey, reaching a flux limit of ∼2 × 10−14 erg s−1 cm−2. Of these, seven are identified as secure X-ray clusters in the soft 0.3–2 keV band using a standard wavelet algorithm on either the PN or the MOS images. Spectroscopic or photometric redshifts are available for five clusters, spanning a range between 0.12 and 0.68. The X-ray spectral fittings show temperatures between 1 and 4.6 keV, characteristic of poor clusters and groups of galaxies. We derive for the first time the XMM—Newton cluster number count log N—log S distribution albeit with poor statistics. Both the log N—log S and the luminosity—temperature relation are in good agreement with previous ROSAT results.

#### The XMM-Newton/2dF Survey — VI. Clustering and bias of the soft X-ray point sources

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, M. Plionis, A. Georgakakis, I. Georgantopoulos Author Notes
Monthly Notices of the Royal Astronomical Society, Volume 356, Issue 1, January 2005, Pages 183–191
Publication year: 2005

## Summary

We study the clustering properties of X-ray sources detected in the wide area (∼2 deg2) bright, contiguous XMM-Newton/2dF survey. We detect 432 objects to a flux limit of 5 × 10−15 erg cm−2 s−1 in the soft 0.5-2 keV band. Performing the standard angular correlation function analysis, a ∼3s correlation signal between 0 and 150 arcsec is detected: w(θ < 150 arcsec) ≃ 0.114 ± 0.037. If the angular correlation function is modelled as a power law, w(θ) = (θ0/θ)≃-1, then for its nominal slope of ≃ = 1.8 we estimate, after correcting for the integral constraint and the amplification bias, that θ0 ≃ 10.4 ± 1.9 arcsec. Very similar results are obtained for the 462 sources detected in the total 0.5-8 keV band (θ0 ≃ 10.8 ± 1.9 arcsec).

Using a clustering evolution model which is constant for comoving coordinates (∈ = -1.2), a luminosity-dependent density evolution (LDDE) model for the X-ray luminosity function and the concordance cosmological model (Ωm = 1 -ΩΛ = 0.3) we obtain, by inverting Limber’s integral equation, a spatial correlation length of r0∼ 16 h−1 Mpc. This value is larger than that of previous ROSAT surveys as well as of the optical two-degree quasar redshift survey. Only in models where the clustering remains constant for physical coordinates (∈ = -3), do we obtain an r0 value (∼7.5 h−1 Mpc) which is consistent with the above surveys.

Finally, comparing the measured angular correlation function with the predictions of the concordance cosmological model, we find for two different bias evolution models that the soft X-ray sources at the present time should be biased with respect to the underline matter fluctuation field with bias values in the range (which depends on the biasing model used): 1.9 ≲b0≲ 2.7 for ∈ = -1.2 or 1 ≲b0≲ 1.6 for ∈ = -3.

#### Cosmological constraints from X-ray AGN clustering and Type Ia supernova data

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, M. Plionis
Monthly Notices of the Royal Astronomical Society: Letters, Volume 360, Issue 1, June 2005, Pages L35–L38
Publication year: 2005

## Summary

We put constraints on the main cosmological parameters of different spatially flat cosmological models by combining the recent clustering results of XMM–Newton soft (0.5–2 keV) X-ray sources, which have a redshift distribution with median redshift z ∼ 1.2, and Type Ia supernova data. Using a likelihood procedure we find that the model that best reproduces the observational data and which is consistent with stellar ages is the concordance λ cold dark matter model with Ω m ≃ 0.28, w ≃ −1, H0 ≃ 72 km s-1 Mpc−1 and t0 ≃ 13.5 Gyr, and has an X-ray active galactic nucleus clustering evolution which is constant in physical coordinates. For a different clustering evolution model (constant in comoving coordinates) we find another viable model, although less probable because of the smaller age of the universe, with Ωm ≃ 0.38, w ≃−1.25, H0 ≃ 70 km s−1 Mpc−1 and t0 ≃ 12.9 Gyr.

#### The XMM–Newton/2dF survey – II. The nature of X-ray-faint optically bright X-ray sources

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
A. Georgakakis, I. Georgantopoulos, M. Vallbé, V. Kolokotronis, S. Basilakos, M. Plionis, G. C. Stewart, T. Shanks, B. J. Boyle
Monthly Notices of the Royal Astronomical Society, Volume 349, Issue 1, March 2004, Pages 135–145
Publication year: 2004

## Summary

In this paper we investigate the properties of low X-ray-to-optical flux ratio sources detected in a wide-area (2.5 deg2) shallow [fX(0.5–8 keV) ≈ 10−14 erg s−1 cm−2]XMM–Newton survey. We find a total of 26 sources (5 per cent of the total X-ray-selected population) with log fX/fopt < −0.9 to the above flux limit. Optical spectroscopy is available for 20 of these low X-ray-to-optical flux ratio objects. Most of them are found to be associated with Galactic stars (a total of eight) and broad-line active galactic nuclei (AGNs; a total of eight). We also find two sources with optical spectra showing absorption and/or narrow emission lines and X-ray/optical properties suggesting AGN activity. Another two sources are found to be associated with low-redshift galaxies with narrow emission-line optical spectra, X-ray luminosities LX(0.5–8 keV) ≈ 1041 erg s−1 and log fX/fopt≈−2 suggesting ‘normal’ star-forming galaxies. Despite the small-number statistics the sky density of ‘normal’ X-ray-selected star-forming galaxies at the flux limit of the present sample is low, consistent with previous ROSAT High-Resolution Imager (HRI) deep surveys. Also, the number density estimated here is in good agreement with both the log N–log S of ‘normal’ galaxies in the Chandra Deep Field North (extrapolated to bright fluxes) and model predictions based on the X-ray luminosity function of local star-forming galaxies.

#### The XMM-Newton/2dF survey - III. Comparison between optical and X-ray cluster detection methods

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, M. Plionis, A. Georgakakis, I. Georgantopoulos, T. Gaga, V. Kolokotronis, G. C. Stewart
Monthly Notices of the Royal Astronomical Society, Volume 351, Issue 3, July 2004, Pages 989–996
Publication year: 2004

## Summary

We directly compare X-ray and optical techniques of cluster detection by combining Sloan Digital Sky Survey photometric data with a wide-field (∼1.6 deg2XMM-Newton survey near the North Galactic Pole region. The optical cluster detection procedure is based on merging two independent selection methods: a smoothing + percolation technique and a matched filter algorithm. The X-ray cluster detection is based on a wavelet-based algorithm, incorporated in the Science Analysis System (SAS) v.5.3 package. The final optical sample counts nine candidate clusters with estimated Automatic Plate Measuring like richness of more than 20 galaxies, while the X-ray based cluster candidates total four. Three out of these four X-ray cluster candidates are also optically detected. We argue that the cause is that the majority of the optically detected clusters are relatively poor X-ray emitters, with X-ray fluxes fainter than the flux limit (for extended sources) of our survey, fx (0.3-2 keV)2 × 10-14 erg cm-2 s-1.

#### The shape of poor groups of galaxies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
M. Plionis, S. Basilakos, H. M. Tovmassian
Monthly Notices of the Royal Astronomical Society, Volume 352, Issue 4, August 2004, Pages 1323–1328
Publication year: 2004

## Summary

We estimate the distribution of intrinsic shapes of UZC–SSRS2 groups of galaxies from the distribution of their apparent shapes. We measure the projected group axial ratio using the moments of their discrete galaxy distribution. Then, using the non-parametric kernel method to estimate the smooth apparent axial ratio distribution, we numerically invert a set of integral equations to recover the corresponding intrinsic distribution under the assumption that groups are either oblate or prolate spheroids. We find that the prolate spheroidal model fits very well the UZC–SSRS2 group distribution with a true mean axial ratio 〈β〉≃ 0.3 and σβ≃ 0.15. This shows that groups of galaxies are significantly more elongated, both on the plane of the sky and in three dimensions, than clusters of galaxies. The poorest groups that we consider, those with four members, are even more elongated than the overall population with 85 per cent of the groups having β≲ 0.4.

#### The Needles in the Haystack Survey: searching for X‐ray‐selected normal galaxies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
A. E. Georgakakis, I. Georgantopoulos, S. Basilakos, M. Plionis, V. Kolokotronis
Monthly Notices of the Royal Astronomical Society, Volume 354, Issue 1, October 2004, Pages 123–126
Publication year: 2004

## Summary

In this paper we present the first results from an ongoing serendipitous survey aiming to identify X‐ray‐selected ‘normal’ galaxies [i.e. not dominated by active galactic nuclei (AGNs)] by combining archival XMM–Newton data with the Sloan Digital Sky Survey. In the first 4.5 deg2 of this program, we have identified a total of 11 ‘normal’ galaxy candidates (eight of them with optical spectroscopy) with fluxes fX(0.5–8 keV) ≈ 10−15–10−13 erg s−1 cm−2. These sources are selected to have low X‐ray‐to‐optical flux ratio (log fX/fopt≲−2), soft X‐ray spectral properties and optical spectra, when available, consistent with the presence of a stellar ionizing continuum. These sources comprise both early‐ and late‐type systems at redshifts z≲ 0.2 with luminosities LX(0.5–8 keV) ≈ 1039–1042 erg s−1. This data set provides the first tight constraint on the surface density of X‐ray‐selected ‘normal’ galaxies at relatively bright fluxes spanning 2 orders of magnitude (10−15–10−13 erg s−1 cm−2). The slope of the ‘normal’ galaxy log N–log S relation in the above flux range is estimated to be −1.4 ± 0.3, consistent with the Euclidean prediction. We also discuss the prospects of ‘normal’ galaxy studies at X‐ray wavelengths using both our continuously expanding survey and future X‐ray missions.

#### The Clustering of XMM-Newton Hard X-Ray Sources

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, A. Georgakakis, M. Plionis, and I. Georgantopoulos
The Astrophysical Journal, 607:L79–L82, 2004 June 1 2004
Publication year: 2004

## Summary

This Letter presents the clustering properties of hard (2-8 keV) X-ray-selected sources detected in a wide-field (≈2 deg2), shallow [fX(2-8 keV) ≈ 10-14 ergs cm-2 s-1], and contiguous XMM-Newton survey. We perform an angular correlation function analysis using a total of 171 sources to the above flux limit. We detect an ~4 σ correlation signal out to 300” with w(θ < 300”)  0.13 ± 0.03. Modeling the two-point correlation function as a power law of the form w(θ) = (θ0/θ)γ-1, we find θ0 = 48.9 arcsec and γ = 2.2 ± 0.30. Fixing the correlation function slope to γ = 1.8, we obtain θ0 = 22.2 arcsec. Using Limber’s integral equation and a variety of possible luminosity functions of the hard X-ray population, we find a relatively large correlation length, ranging from r0 ~ 9 to 19 h-1 Mpc (for γ = 1.8 and the concordance cosmological model), with this range reflecting also different evolutionary models for the source luminosities and clustering characteristics. The relatively large correlation length is comparable to that of extremely red objects and luminous radio sources.

#### On the Lx − σv relation of groups of galaxies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis, H. M. Tovmassian
A&A Volume 416, Number 2, March III 2004
Publication year: 2004

## Summary

We analyse the  relation for the new Mulchaey et al. group Atlas. We find that once we take into account the possible statistical bias introduced by the cutoff in luminosity, we recover a relation that is consistent with that of clusters, i.e., . The larger scatter of this relation for groups of galaxies could be attributed to an orientation effect, due to which the radial velocity dispersion of groups oriented close to orthogonal to the line of sight, would be underestimated. This effect could also contribute to flattening the slope of the group  relation.

#### Modelling the two-point correlation function of galaxy clusters in the Sloan Digital Sky Survey

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Spyros Basilakos, Manolis Plionis
Monthly Notices of the Royal Astronomical Society, Volume 349, Issue 3, April 2004, Pages 882–888
Publication year: 2004

## Summary

We study the clustering properties of the recently compiled Sloan Digital Sky Survey (SDSS) cluster catalogue using the two-point correlation function in redshift space. We divide the total SDSS sample into two richness subsamples, roughly corresponding to Abell R≥ 0 and Automated Plate Measuring (APM) clusters, respectively. If the two-point correlations are modelled as a power law, ξ(r) = (r0/r)γ, then the best-fitting parameters for the two subsamples are r0= 20.7+4.0−3.8h−1 Mpc with γ= 1.6+0.4−0.4 and r0= 9.7+1.2−1.2 with γ= 2.0+0.7−0.5h−1 Mpc, respectively. Our results are consistent with the dependence of cluster richness on the cluster correlation length.

Finally, comparing the SDSS cluster correlation function with predictions from three flat cosmological models (Ωm= 0.3) with dark energy (quintessence), we estimate the cluster redshift-space distortion parameter β≃Ω0.6m/b0 and the cluster bias at the present time. For the Λ cold dark matter case we find β= 0.2+0.029−0.016, which is in agreement with the results based on large-scale cluster motions.

#### Galaxy Bias in Quintessence Cosmological Models

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, M. Plionis
The Astrophysical Journal, 593:L61-L64, 2003 August 20 © 2003
Publication year: 2003

## Summary

We derive the evolution of the linear bias factor, b(z), in cosmological models driven by an exotic fluid with an equation of state px wx, where -1  w < 0 (quintessence). Our aim is to put constrains on different cosmological and biasing models by combining the recent observational clustering results of optical (2dF) galaxies (Hawkins et al.) with those predicted by the models. We find that when fitted to the 2dF clustering results, our bias model predicts different bias evolution for different values of w. The models that provide the weak biasing (b0  1.1) of optical galaxies found in many recent observational studies are flat, m = 0.3 with w  -0.9. These models, however, predict a weak redshift evolution of b(z), not corroborated by N-body simulations.

#### Galaxy Alignments as a Probe of the Dynamical State of Clusters

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
M. Plionis, C. Benoist, S. Maurogordato, C. Ferrari, and S. Basilakos
The Astrophysical Journal, 594:144-153, 2003 September 1
Publication year: 2003

## Summary

We present indications, based on a sample of 303 Abell clusters, for a relation between the dynamical state of clusters and the alignments of galaxy members with their parent cluster major-axis orientation as well as with the large-scale environment within which the clusters are embedded. The statistical results are complemented with a deep, wide-field case study of galaxy alignments in the cluster A521, which is characterized by multiple merging events (Maurogrdato et al.; Ferrari et al.) and whose galaxy members show a strong alignment signal out to h-1 Mpc. Our results show that galaxy alignments appear to be stronger the more dynamically young the cluster is, especially when found in high-density environments. This relation complements the recently found “cluster substructurealignment connection” (Plionis & Basilakos) by which dynamically young clusters, found in high-density environments, show stronger cluster-cluster alignments.

#### The size and shape of local voids

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis, Spyros Basilakos
Monthly Notices of the Royal Astronomical Society, Volume 330, Issue 2, February 2002, Pages 399–404
Publication year: 2002

## Summary

We study the size and shape of low-density regions in the local Universe, which we identify in the smoothed density field of the PSCz flux-limited IRAS galaxy catalogue. After quantifying the systematic biases that enter the detection of voids using our data set and method, we identify, using a smoothing length of 5 h−1 Mpc, 14 voids within 80 h−1 Mpc (having volumes ⩾103h−3 Mpc3) and, using a smoothing length of 10 h−1 Mpc, eight voids within 130 h−1 Mpc (having volumes ⩾8×103 h−3 Mpc3). We study the void size distribution and morphologies and find that there is roughly an equal number of prolate and oblate-like spheroidal voids. We compare the measured PSCz void shape and size distributions with those expected in six different cold dark matter (CDM) models and find that only the size distribution can discriminate between models. The models preferred by the PSCz data are those with intermediate values of σ8(≃0.83), independent of cosmology.

#### The cluster substructure—alignment connection

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis, Spyros Basilakos
Monthly Notices of the Royal Astronomical Society, Volume 329, Issue 2, January 2002, Pages L47–L51
Publication year: 2002

## Summary

Using a sample of 903 APM clusters we investigate whether their dynamical status, as evidenced by the presence of significant substructures, is related to the large scale structure of the Universe. We find that the cluster dynamical activity is strongly correlated with the tendency of clusters to be aligned with their nearest neighbour and in general with the nearby clusters that belong to the same supercluster. Furthermore, dynamically active clusters are more clustered than the overall cluster population. These are strong indications that clusters develop in a hierarchical fashion by anisotropic merging along the large scale filaments within which they are embedded.

#### Supercluster properties as a cosmological probe

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
V. Kolokotronis, S. Basilakos, M. Plionis
Monthly Notices of the Royal Astronomical Society, Volume 331, Issue 4, April 2002, Pages 1020–1026
Publication year: 2002

## Summary

We investigate the supercluster shape properties of the all-sky observed (Abell/ACO) and simulated (Virgo data) cluster catalogues using an approach based on differential geometry. We identify rich superclusters by applying the percolation algorithm to both observed and mock cluster populations, the latter being constructed following the observational selection of the Abell/ACO sample, extended out to zmax≤0.114. We apply a set of shape diagnostics in order to study the morphological features of superclusters with ≥8 cluster members. Our results demonstrate that filamentarity is the dominant supercluster shape feature. On comparing data and models, we show that the ΛCDM (ΩΛ=1-Ωm=0.7) model performs better than τCDM, which is excluded at a relatively high confidence level, in agreement with other recent large-scale structure studies.

#### Recent Dynamical Evolution of Galaxy Clusters

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis
The Astrophysical Journal, 572:L67-L70, 2002 June 10
Publication year: 2002

## Summary

Evidence is presented for the recent evolution of the relaxation processes in clusters of galaxies, using large optical and X-ray cluster samples. The criteria of the cluster relaxation used are the cluster ellipticity, the intracluster medium (ICM) temperature, and X-ray cluster luminosity. We find evidence of varying strength and significance of all three indicators evolving with redshift for z  0.15. This result supports the view that clusters have mostly stopped undergoing mergers and accreting matter, as expected in a low-m universe, and are now in the process of gravitational relaxation, which reduces their flattening, their ICM temperature (shock heated during the merging phase), and their X-ray luminosity. These results support similar recent claims of Melott, Chambers, & Miller.

#### Searching for cluster substructure using APM and ROSAT data

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
V. Kolokotronis, S. Basilakos, M. Plionis, I. Georgantopoulos
Monthly Notices of the Royal Astronomical Society, Volume 320, Issue 1, January 2001, Pages 49–60
Publication year: 2001

## Summary

We present a detailed study of the morphological features of 22 rich galaxy clusters. Our sample is constructed from a cross-correlation of optical (Abell + APM) data with X-ray (0.1–2.4 keV) ROSAT pointed observations. We systematically compare cluster images and morphological parameters in an attempt to reliably identify possible substructure in both optical and the X-ray images. To this end, we compute various moments of the optical and X-ray surface-brightness distribution such as the ellipticities, centre-of-mass shifts and ellipsoidal orientations. We assess the significance of our results using Monte Carlo simulations. We find significant correlations between the optical and X-ray morphological parameters, indicating that in both parts of the spectrum it is possible to identify correctly the dynamical state of a cluster. Most of our clusters (17/22) have a good one-to-one correspondence between the optical and the X-ray images, and about 10 appear to have strong indications of substructure. This corresponds to a minimum percentage of order ∼45 per cent, which is in very good accordance with other similar analyses. Finally, five out of 22 systems (∼22 per cent) seem to have distinct subclumps in the optical which are not verified in the X-ray images, and thus are suspect of being due to optical projection effects. These results will serve as a useful guide in interpreting subsequent analyses of large optical cluster catalogues.

#### PSCz superclusters: detection, shapes and cosmological implications

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, M. Plionis, M. Rowan-Robinson
Monthly Notices of the Royal Astronomical Society, Volume 323, Issue 1, May 2001, Pages 47–55
Publication year: 2001

## Summary

We study the possibility of correctly identifying, from the smooth galaxy density field of the PSCz flux-limited catalogue, high-density regions (superclusters) and recovering their true shapes in the presence of a bias introduced by the coupling between the selection function and the constant radius smoothing. We quantify such systematic biases in the smoothed PSCz density field and after applying the necessary corrections we study supercluster multiplicity and morphologies using a differential geometry definition of shape. Our results strongly suggest that filamentary morphology is the dominant feature of PSCz superclusters. Finally, we compare our results with those expected in three different cosmological models and find that the Λ cold dark matter (CDM) model Λ=1−Ωm=0.7) performs better than Ωm=1 CDM models.

#### Cosmological implications of the PSCz PDF and its moments

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis, Spyros Basilakos
Monthly Notices of the Royal Astronomical Society, Volume 327, Issue 2, October 2001, Pages L32–L36
Publication year: 2001

## and

one COBE-normalized, and the old standard cold dark matter model. The galaxy distributions are smoothed with a Gaussian window at three different smoothing scales,

10 and 15 h−1 Mpc. We find that the simulation PSCz look-alike PDFs are sensitive only to the normalization of the power spectrum, probably owing to the shape similarity of the simulated galaxy power spectrum on the relevant scales. We find that the only models that are consistent, at a high significance level, with the observed PSCz PDF are models with a relatively low power spectrum normalization

From the phenomenologically derived σ8–moments relation, fitted from the simulation data, we find that the PSCz moments suggest

.

#### Cosmological Evolution of Linear Bias

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, M. Plionis
The Astrophysical Journal, 550:522-527, 2001 April 1
Publication year: 2001

## Summary

Using linear perturbation theory and the Friedmann-Lemaitre solutions of the cosmological field equations, we analytically derive a second-order differential equation for the evolution of the linear bias factor, b(z), between the background matter and a mass-tracer fluctuation field. We find b(z) to be a strongly dependent function of redshift in all cosmological models. Comparing our analytical solution with the semianalytic model of Mo & White, which utilizes the Press-Schechter formalism and gravitationally induced evolution of clustering, we find an extremely good agreement even at large redshifts, once we normalize to the same bias value at two different epochs, one of which is the present. Furthermore, our analytic b(z) function agrees well with the outcome of N-body simulations.

#### The apparent and intrinsic shape of the APM galaxy clusters

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Basilakos, M. Plionis, S. J. Maddox
Monthly Notices of the Royal Astronomical Society, Volume 316, Issue 4, August 2000, Pages 779–785
Publication year: 2000

## Summary

We estimate the distribution of intrinsic shapes of APM galaxy clusters from the distribution of their apparent shapes. We measure the projected cluster ellipticities using two alternative methods. The first method is based on moments of the discrete galaxy distribution while the second is based on moments of the smoothed galaxy distribution. We study the performance of both methods using Monte Carlo cluster simulations covering the range of APM cluster distances and including a random distribution of background galaxies. We find that the first method suffers from severe systematic biases, whereas the second is more reliable. After excluding clusters dominated by substructure and quantifying the systematic biases in our estimated shape parameters, we recover a corrected distribution of projected ellipticities. We use the non-parametric kernel method to estimate the smooth apparent ellipticity distribution, and numerically invert a set of integral equations to recover the corresponding distribution of intrinsic ellipticities under the assumption that the clusters are either oblate or prolate spheroids. The prolate spheroidal model fits the APM cluster data best.

#### The angular correlation function of the ROSAT All-Sky Survey Bright Source Catalogue

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
A. Akylas, I. Georgantopoulos, M. Plionis
Monthly Notices of the Royal Astronomical Society, Volume 318, Issue 4, November 2000, Pages 1036–1040
Publication year: 2000

## Summary

We have derived the angular correlation function of a sample of 2096 sources detected in the ROSAT All-Sky Survey (RASS) Bright Source Catalogue, in order to investigate the clustering properties of active galactic nuclei (AGN) in the local Universe. Our sample is constructed by rejecting all known stars, as well as extended X-ray sources. Areas with |b|<30° and declination δ<-30° are also rejected owing to the high or uncertain neutral hydrogen absorption. Cross-correlation of our sample with the Hamburg/RASS optical identification catalogue suggests that the vast majority of our sources are indeed AGN. A 4.1σ correlation signal between 0° and 8° was detected with w(θ<8°)=2.5±0.6×10−2. Assuming a two-point correlation function of the form w(θ)=(θθ0)−0.8, we find θ0=0.°062. Deprojection on three dimensions, using Limber’s equation, yields a spatial correlation length of r0≈6.0±1.6 h−1 Mpc. This is consistent with the AGN clustering results derived at higher redshifts in optical surveys and suggests a comoving model for the clustering evolution.

#### Galaxy and cluster biasing from Local Group dynamics

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
M. Plionis, S. Basilakos, M. Rowan-Robinson, S. J. Maddox, S. J. Oliver, O. Keeble, W. Saunders
Monthly Notices of the Royal Astronomical Society, Volume 313, Issue 1, March 2000, Pages 8–12
Publication year: 2000

## Summary

Comparing the gravitational acceleration induced on the Local Group of galaxies by different tracers of the underlying density field we estimate, within the linear gravitational instability theory and the linear biasing ansatz, their relative bias factors. Using optical SSRS2 galaxies, IRAS (PSCz) galaxies and Abell/ACO clusters, we find bO,I ≈ 1.21±0.06 and bC,I≈4.3±0.8, in agreement with other recent studies. Finally, there is an excellent one-to-one correspondence of the PSCz and Abell/ACO cluster dipole profiles, once the latter is rescaled by bC,I, out to at least ∼150 h−1 Mpc.

#### Cluster versus POTENT density and velocity fields: cluster biasing and Ω

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
E. Branchini, I. Zehavi, M. Plionis, A. Dekel
Monthly Notices of the Royal Astronomical Society, Volume 313, Issue 3, April 2000, Pages 491–503
Publication year: 2000

## Summary

The density and velocity fields as extracted from the Abell/ACO clusters are compared with the corresponding fields recovered by the POTENT method from the Mark III peculiar velocities of galaxies. In order to minimize non-linear effects and to deal with ill-sampled regions, we smooth both fields using a Gaussian window with radii ranging between 12 and 20 h−1 Mpc. The density and velocity fields within 70 h−1 Mpc exhibit similarities, qualitatively consistent with gravitational instability theory and a linear biasing relation between clusters and mass. The random and systematic errors are evaluated with the help of mock catalogues. Quantitative comparisons within a volume containing ∼12 independent samples yield βc≡Ω0.6bc=0.22±0.08, where bc is the cluster biasing parameter at 15 h−1 Mpc. If bc∼4.5, as indicated by the cluster correlation function, our result is consistent with Ω ∼ 1.

#### The X-ray luminosity function of local galaxies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
I. Georgantopoulos, S. Basilakos, And M. Plionis
Monthly Notices of the Royal Astronomical Society, Volume 305, Issue 1, May 1999, Pages L31–L34
Publication year: 1999

## Summary

We present an estimate of the local X-ray luminosity function and emissivity for different subsamples of galaxies, namely Seyferts, LINERS, star-forming and passive (no-emission-line) galaxies. This is performed by convolving their optical luminosity function, as derived from the Ho et al. spectroscopic sample of nearby galaxies with the corresponding Lx/LB relation. The local galaxy emissivity is ≈ 1.6 × 1039 h erg s−1 Mpc−3 in agreement with the results from a number of cross-correlation analyses using large-area surveys. From our analysis, it becomes evident that the largest fraction of the galaxy emissivity comes from galaxies associated with active galactic nuclei (Seyferts but also LINERS) while the contribution of star-forming and passive galaxies is small. This independently supports the view that most of the yet unidentified X-ray sources in deep ROSAT fields which are associated with faint optical galaxies do harbour an active galactic nucleus.

#### The ROSAT X-ray background dipole

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
M. Plionis, I. Georgantopoulos
Monthly Notices of the Royal Astronomical Society, Volume 306, Issue 1, June 1999, Pages 112–116
Publication year: 1999

## Summary

We estimate the dipole of the diffuse 1.5-keV X-ray background from the ROSAT all-sky survey map of Snowden et al. We first subtract the diffuse Galactic emission by fitting an exponential scaleheight, finite-radius, disc model to the data. We further exclude regions of low galactic latitudes, of local X-ray emission (e.g. the North Polar Spur) and model them using two different methods. We find that the ROSAT X-ray background dipole points towards (l,b) ≈ (288° 25°) ± 19° in consistency with the cosmic microwave background (within ∼ 30°); its direction is also in good agreement with the HEAO-1 X-ray dipole at harder energies. The normalized amplitude of the ROSAT XRB dipole is ∼ 1.7 per cent. Subtracting from the ROSAT map the expected X-ray background dipole resulting from the reflex motion of the observer with respect to the cosmic rest frame (Compton-Getting effect) we find the large-scale dipole of the X-ray emitting extragalactic sources having an amplitude DLSS∼ 0.9 DXRB, in general agreement with the predictions of Lahav et al. We finally estimate that the Virgo cluster is responsible for ∼ 20 per cent of the total measured XRB dipole amplitude.

#### Cosmological constraints from the clustering properties of the X-ray Brightest Abell-type Cluster sample

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Borgani, M. Plionis, V. Kolokotronis
Monthly Notices of the Royal Astronomical Society, Volume 305, Issue 4, May 1999, Pages 866–874
Publication year: 1999

## Summary

We present an analysis of the two-point correlation function, ξ (r), of the X-ray Brightest Abell-type Cluster sample (XBACs) of Ebeling et al. and of the cosmological constraints that it provides. If ξ (r) is modelled as a power-law, ξ (r)=(r0/r)γ, we find r0 ≃ 26.0 ± 4.5 h−1 Mpc and γ≃ 2.0 ± 0.4, with errors corresponding to 2σ uncertainties for one significant fitting parameter. As a general feature, ξ (r) is found to remain positive up to r ≃ 50–55 h−1 Mpc, after which it declines and crosses zero. Only a marginal increase of the correlation amplitude is found as the flux limit is increased from 5 × 10−12 to 12 × 10−12 erg s−1 cm−2, thus indicating a weak dependence of the correlation amplitude on the cluster X-ray luminosity. Furthermore, we present a method to predict correlation functions for flux-limited X-ray cluster samples from cosmological models. The method is based on the analytical recipe by Mo & White and on an empirical approach to convert cluster fluxes into masses. We use a maximum likelihood method to place constraints on the model parameter space from the XBACs ξ (r). For scale-free primordial spectra, we find that the shape parameter of the power spectrum is determined to lie in the 2σ range 0.05 ≲Γ≲ 0.20. As for the amplitude of the power spectrum, we find σ8≃ 0.4–0.8 for Ω0=1 and σ8≃ 0.8–2.0 for Ω0=0.3. The latter result is in complete agreement with, although less constraining than, results based on the local cluster abundance.

#### The X-Ray Cluster Dipole

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Plionis M., Kolokotronis V.
The Astrophysical Journal, Volume 500, Issue 1, pp. 1-7
Publication year: 1998

## Summary

We estimate the dipole of the whole-sky, X-ray flux-limited sample of Abell/Abell, Corwin, & Olowin (ACO) clusters (XBACs) and compare it to the optical cluster dipole, which is known to be well aligned with the cosmic microwave background (CMB) dipole and which converges to its final value at ~160 h-1 Mpc. We find the X-ray cluster dipole to be well aligned (<~25°) with the CMB dipole, while it closely follows the radial profile of its optical cluster counterpart, although its amplitude is about 10%-30% lower. In view of the fact that the XBAC sample is not affected by the volume incompleteness and the projection effects that are known to exist at some level in the optical parent Abell/ACO cluster catalog, our present results confirm the previous optical cluster dipole analysis that there are significant contributions to the Local Group motion from large distances (~160 h-1 Mpc). In order to assess the expected contribution to the X-ray cluster dipole from a sample that is closer to being purely X-ray-selected, we compare the dipoles of the XBACs and the brightest-cluster sample in their overlap region. The resulting dipoles are in good mutual agreement with an indication that the XBAC sample slightly underestimates the full X-ray dipole (by <~5%), while the Virgo Cluster contributes about 12% to the overall X-ray cluster dipole. Using linear perturbation theory to relate the X-ray cluster dipole to the Local Group peculiar velocity, we estimate βcX (≡Ω0.60/bcX)~=0.24+/-0.05.

#### The cluster distribution as a test of dark matter models – IV. Topology and geometry

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
P. Coles, R. C. Pearson, S. Borgani, M. Plionis, L. Moscardini
Monthly Notices of the Royal Astronomical Society, Volume 294, Issue 2, February 1998, Pages 245–258
Publication year: 1998

## Summary

We study the geometry and topology of the large-scale structure traced by galaxy clusters in numerical simulations of a box of side 320 h-1 Mpc, and compare them with available data on real clusters. The simulations we use are generated by the Zel’dovich approximation, using the same methods as we have used in the first three papers in this series. We consider the following models to see if there are measurable differences in the topology and geometry of the superclustering they produce: (i) the standard cold dark matter model (SCDM); (ii) a CDM model with Ω0 = 0.2 (OCDM); (iii) a CDM model with a ‘tilted’ power spectrum having n = 0.7 (TCDM); (iv) a CDM model with a very low Hubble constant, h = 0.3 (LOWH); (v) a model with mixed CDM and HDM (CHDM); (vi) a flat low-density CDM model with Ω0 = 0.2 and a non-zero cosmological Λ term (ΛCDM). We analyse these models using a variety of statistical tests based on the analysis of: (i) the Euler—Poincaré characteristic; (ii) percolation properties; (iii) the minimal spanning tree construction. Taking all these tests together we find that the best-fitting model is ΛCDM and, indeed, the others do not appear to be consistent with the data. Our results demonstrate that despite their biased and extremely sparse sampling of the cosmological density field, it is possible to use clusters to probe subtle statistical diagnostics of models, which go far beyond the low-order correlation functions usually applied to study superclustering.

#### Modelling the X-ray cluster dipole and cluster contribution to the soft X-ray background

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
V. Kolokotronis, M. Plionis, P. Coles, S. Borgani
Monthly Notices of the Royal Astronomical Society, Volume 295, Issue 1, March 1998, Pages 19–32
Publication year: 1998

## Summary

We investigate the sampling and dipole convergence properties of flux-limited samples of mock X-ray clusters in relation to their underlying ‘parent’ cluster distribution. To this purpose, we resort to numerical simulations of the cluster distribution and extract samples resembling the main observational features of X-ray selected cluster samples. The flux-limited samples, being quite sparse, underestimate the amplitude of the ‘parent’ cluster dipole by ≈ 15 per cent on average for Local Group-like observers. However, the general shapes of their dipole amplitude profiles are in relatively good agreement. We also calculate the expected contribution of clusters, selected according to the relevant criteria, to the soft (i.e. 0.1–2.4 keV) extragalactic X-ray background (XRB), using the ESO Key Project X-ray luminosity function, assuming a flat universe with vanishing cosmological constant. We obtain a value of about 10 per cent of the observed XRB flux.

#### Large-scale coherent dipole anisotropy?

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S.Basilakos, M.Plionis
Monthly Notices of the Royal Astronomical Society, Volume 299, Issue 3, pp. 637-642
Publication year: 1998

## Summary

We have reanalysed and compared the dipoles of the 1.2-Jy and 0.6-Jy (QDOT) IRAS galaxy samples. We find strong indications from both samples for (a) significant contributions to the
gravitational field that shapes the Local Group motion from depths up to ,170 h¹1 Mpc and (b) a large-scale coherence of the dipole anisotropy, indications provided mainly by the fact that the differential dipoles of large equal-volume shells are aligned with the CMB dipole and exhibit significant dipole signals. The two IRAS dipoles are indistinguishable within 50 h¹1
Mpc and beyond ,130 h¹1 Mpc while the QDOT dipole, having a lower flux limit, continues growing with respect to the 1.2-Jy sample up to ,100 h¹1 Mpc in agreement with Rowan-
Robinson et al.

#### Minkowski functionals of Abell/ACO clusters

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
M. Kerscher, J. Schmalzing, J. Retzlaff, S. Borgani, T. Buchert, S. Gottlöber, V. Müller, M. Plionis, H. Wagner
Monthly Notices of the Royal Astronomical Society, Volume 284, Issue 1, January 1997, Pages 73–84
Publication year: 1997

## Summary

We determine the Minkowski functionals for a sample of Abell/ACO clusters, 401 with measured and 16 with estimated redshifts. The four Minkowski functionals (including the void probability function and the mean genus) deliver a global description of the spatial distribution of clusters on scales from 10 to 60 Mh−1 Mpc with a clear geometric interpretation. Comparisons with mock catalogues of N-body simulations using different variants of the CDM model demonstrate the discriminative power of the description. The standard CDM model and the model with a tilted perturbation spectrum cannot generate the Minkowski functionals of the cluster data, while a model with a cosmological constant and a model with breaking of the scale invariance of perturbations (BSI) yield compatible results.

#### Constraining the power spectrum using clusters

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
S. Borgani, L. Moscardini, M. Plionis, K.M. Gorski, J. Holtzman, A. Klypin, J.R. Primack, C.C. Smith, R. Stompor
New Astronomy 1 (1997) p.321-347
Publication year: 1997

## Summary

We analyze an extended redshift sample of Abell/ACO clusters and compare the results with those coming from numerical simulations of the cluster distribution, based on the truncated Zel’dovich approximation (TZA), for a list of eleven dark matter (DM) models. For each model we run several realizations, so that we generate a set of 48 independent mock Abell/ACO cluster samples per model, on which we estimate cosmic variance effects. Other than the standard CDM model, we consider (a) Ω0 = 1 CDM models based on lowering the Hubble parameter and/or on tilting the primordial spectrum; (b) Ω0 = 1 Cold + Hot DM models with 0.1 ≤Ων ≤0.5; (c) low-density flat ΛCDM models with 0.3 ≤Ω0 ≤0.5. We compare real and simulated cluster distributions by analysing correlation statistics, the probability density function, and supercluster properties from percolation analysis. We introduce a generalized definition of the spectrum shape parameter Γ in terms of σ25/σ8, where σris the rms fluctuation amplitude within a sphere of radius r. As a general result, we find that the distribution of galaxy clusters provides a constraint only on the shape of the power spectrum, but not on its amplitude: a shape parameter 0.18 ≲ Γ ≲ 0.25 and an effective spectral index at the 20 h−1 Mpc scale −1.1 ≲ neff ≲ −0.9 are required by the Abell/ACO data. In order to obtain complementary constraints on the spectrum amplitude, we consider the cluster abundance as estimated using the Press-Schechter approach, whose reliability is explicitly tested against N-body simulations. By combining results from the analysis of the distribution and the abundance of clusters we conclude that, of the cosmological models considered here, the only viable models are either Cold + Hot DM ones with 0.2 ≲ Ων ≲ 0.3, better if shared between two massive ν species, and ΛCDM ones with 0.3 ≲Ω0≲0.5.

#### The cluster distribution as a test of dark matter models — III. The cluster velocity field

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
L. Moscardini, E. Branchini, P. Tini Brunozzi, S. Borgani, M. Plionis, P. Coles
Monthly Notices of the Royal Astronomical Society, Volume 282, Issue 2, September 1996, Pages 384–400
Publication year: 1996

## Summary

We study the large-scale velocity fields traced by galaxy clusters in numerical simulations of a box of side 960 hs−1 Mpc, and compare them with available data on real clusters. In order to test the reliability of the simulations, which are based on an optimized version of the Zel’dovich approximation, we compare their cluster velocities with those of ‘exact’ N-body simulations, and find a remarkable agreement between the two according to a variety of statistical tests. We analyse cold dark matter (CDM) models with density parameter in the range 0.2 ⩽ Ω0 ⩽ 1, both with and without the cosmological constant term to provide a flat geometry. We also simulate a cold+hot dark matter (CHDM) model, with 30 per cent provided by the hot component. Comparison with real data is performed by applying tests based on the cumulative velocity frequency distribution (CVFD) and bulk flow statistics. For the CVFD, we use observational velocity data from different authors, and find that results based on different data sets are contradictory. In particular, the recent infrared Tully−Fisher (IRTF) data of Giovanelli yield smaller velocities with smaller errors than both the IRTF and Dns−σ data of Hudson. It turns out that the Giovanelli data are only only consistent with the open Ω0 = 0.4 and the flat Ω0 = 0.2 models, while the Hudson data, though less discriminatory because of their larger errors, appear to exclude open models with Ω0 ⩽ 0.4 and flat models with Ω0 = 0.2. This latter conclusion also holds if one pools all the data into a single sample regardless of the systematic differences in the two different sources. Furthermore, CVFD and bulk flow analyses of the Branchini et al. reconst-ructed velocity data again disfavour precisely those models accepted on the grounds of Giovanelli’s sample. Finally, we confirm that the Lauer & Postman reported bulk flow determination would be a rare event in the cosmological models we have analysed.

#### Sampling effects on cosmological dipoles

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
V. Kolokotronis, M. Plionis, P. Coles, S. Borgani, L. Moscardini
Monthly Notices of the Royal Astronomical Society, vol. 280, p. 186
Publication year: 1996

## Summary

We use numerical simulations to investigate the behaviour of the dipole moment of the spatial distribution of different kinds of mass tracers. We select density peaks of the simulated matter distribution with mean separations of 38 and 30 h1 Mpc to represent two samples of rich clusters of galaxies. We extract, from the same simulations, samples selected to mimic the full 3D galaxy distribution of IR galaxies, and the flux–limited IRAS and QDOT galaxy samples. We compare the dipole moments of these “galaxy” and “cluster” samples in order to assess the effects of sampling uncertainties and shot–noise on the relationship between the “true” underlying galaxy dipole and the dipoles obtained for clusters and for the flux–limited galaxy samples. The results of this analysis demonstrate that the dipoles of both the IRAS and QDOT–like catalogues should trace the full 3D dipole shape fairly accurately, with the loss however of about 15–20% of the total 3D dipole amplitude. Furthermore, using a simple argument based on linear perturbation theory, on the linear biasing assumption and on the amplitude of the cluster dipole relative to that of galaxies, we can estimate their relative biasing factors quite accurately and in agreement with results obtained by other methods.

#### Reconstructing Positions and Peculiar Velocities of Galaxy Clusters within 20000 km/sec. I: The Cluster 3D Dipole

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Branchini Enzo, Plionis Manolis
Astrophysical Journal v.460, p.569
Publication year: 1996

## Summary

Starting from the observed distribution of Abell/ACO galaxy clusters in redshift space, we use a two-step procedure to recover their distances and peculiar velocities. After statistically correcting for the unobserved cluster distribution in the zone of avoidance and also for a smooth absorption at higher latitudes, we use a dynamical iterative algorithm, based on that published by Strauss & Davis in 1988, to recover the real-space cluster positions by minimizing the redshift-space distortions. The whole procedure assumes that clusters trace the mass, that peculiar velocities are caused by gravity, and that linear perturbation theory applies. The amplitude of the cluster dipole measured in real space turns out to be ~23% less than that measured in redshift space. In both cases the dipole direction is aligned with the cosmic microwave background dipole within ~10^deg^, taking into account the Virgocentric infall component of the Local Group motion. Using linear theory we obtain β_c_( = {OMEGA}_0_^0.6^/b_c_) ~ 0.21(+/- 0.03), where the uncertainty is due to observational errors and limitations in the reconstruction procedure, while the intrinsic cosmological variance amounts to ~0.07. This β_c_ value implies that for a cluster-mass bias parameter of b_c_ <~ 5, a flat universe is not excluded, contrary to previous cluster-dipole z-space analysis. A more stringent determination of β_c_ will be obtained from the analysis of the peculiar velocity field in a forthcoming paper.

#### The one-point cluster distribution function and its moments

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis, Riccardo Valdarnini
Monthly Notices of the Royal Astronomical Society, Volume 272, Issue 4, February 1995, Pages 869–877
Publication year: 1995

## Summary

We derive the one-point probability density function (pdf) of the smoothed Abell-ACO cluster density field and we compare it with that of artificial cluster samples, generated as high peaks of a Gaussian field in such a way that they reproduce the low-order (two- and three-point) correlation functions and the observed cluster selection functions. We find that both real and simulated pdfs are well approximated by a log-normal distribution, even when the Gaussian smoothing radius is as large as 40 h1h−1 Mpc. Furthermore, the low-order moments of the pdf are found to obey a relation γ1.8(±0.2)σ4γ≈1.8(±0.2)σ4, with γ being the skewness. Since clusters have not had enough time to depart significantly from their original birth-place positions, these results are consistent with the theory that clusters are high peaks of an underlying initial Gaussian density field.

A by-produce of our analysis is that when we re-scale the pdf cluster moments to those of the QDOT-IRAS galaxies, using linear biasing with bcI∼4.5 and for the common smoothing radius of 20 h1h−1 Mpc, we find them to be significantly smaller than those directly estimated from the QDOT data by Saunders et al.

#### The cluster distribution as a test of dark matter models – II. The dipole structure

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
P. Tini Brunozzi, S. Borgani, M. Plionis, L. Moscardini, P. Coles
Monthly Notices of the Royal Astronomical Society, Volume 277, Issue 4, December 1995, Pages 1210–1224
Publication year: 1995

## Summary

We study the dipole structure of an extended redshift sample of Abell/ACO clusters in order to infer the dynamical origin of the motion of the Local Group (LG). To elucidate further the constraints that this motion places on dark matter models, we use numerical simulations based on an optimized version of the truncated Zel’dovich approximation which we have shown in an accompanying paper to provide a reliable representation of large-scale gravitational clustering. Taking advantage of their low computational cost, we run 20 realizations of each of six different dark matter models: four of these have a density parameter Ω 0 = 1, while the other two have Ω 0= 0.2, one with and one without a cosmological constant term. For the Abell/ACO sample, we have evaluated the parameter β=Ω0.60/bclβ=Ω00.6/bcl(where bcl is the linear bias parameter for the clusters), which reaches its asymptotic value at Rconv160h1MpcRconv≃160h−1Mpc. Convergence occurs at this scale whether calculations are performed in the β LGor in the CMB frame, but the asymptotic value differs in these two cases: we find βLG=0.15 ± 0.04 and βCMB = 0.25 ± 0.06, respectively. After identifying in the simulations those observers having local densities and peculiar velocities similar to those of the Local Group, we construct mock cluster samples around them reproducing the same observational biases, and apply to these mock samples the same method of analysis as we used for the Abell/ACO sample. We find that an alignment between the cluster dipole and observer velocity (‘CMB’ dipole) directions, such as that observed (∆θ≲ 20°), should not be expected necessarily: much larger misalignment angles are often found in all models considered. This, together with the large observer-to-observer variance estimates of β, makes it difficult to place any firm constraints on cosmological models. This result suggests that the dipole analysis of the cluster distribution has a relevance that is cosmographical, rather than cosmological. Our results also demonstrate that the large amplitude and convergence depth of the observed cluster dipole cannot be taken as strong evidence either for or against a lowdensity Universe.

#### The cluster distribution as a test of dark matter models – I. Clustering properties

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Stefano Borgani, Manolis Plionis, Peter Coles, Lauro Moscardini
Monthly Notices of the Royal Astronomical Society, Volume 277, Issue 4, December 1995, Pages 1191–1209
Publication year: 1995

## Summary

We present extended simulations of the large-scale distribution of galaxy clusters in several dark matter models, using an optimized version of the truncated Zel’dovich approximation (TZA). In order to test the reliability of our simulations, we compare them with N-body-based cluster simulations. We find that the TZA provides a very accurate description of the cluster distribution as long as fluctuations on the cluster mass-scale are in the mildly non-linear regime (σ8 ≲ 1). The low computational cost of this simulation technique allows us to run a large ensemble of 50 realizations for each model, so we are able to quantify accurately the effects of cosmic variance. Six different dark matter models are studied in this work: standard CDM (SCDM), tilted CDM (TCDM) with primordial spectral index n = 0.7, cold+hot DM (CHDM) with Ωhot = 0.3, low Hubble constant (h = 0.3) CDM (LOWH), and two spatially flat lowdensity CDM models with Ω0 = 0.2 and ΩΛ = 0.8, having two different normalizations, σ8 = 0.8 (ΛCDM1) and σ8 = 1.3 (ΛCDM2). We compare the cluster simulations with an extended redshift sample of Abell/ACO clusters, using various statistical measures, such as the integral of the two-point correlation function, J3, and the probability density function (pdf). We find that the models that best reproduce the clustering of the Abell/ACO cluster sample are the CHDM and the ΛCDM1 models. The ΛCDM2 model is too strongly clustered, and this clustering is probably overestimated in our simulations as a result of the large σ8-value of this model. All of the other models are ruled out at a high confidence level. The pdfs of all models are well approximated by a lognormal distribution, consistent with similar findings for Abell/ACO clusters. The low-order moments of all the pdfs are found to obey a variance-skewness relation of the form γ ≈ S3σ4, with S3 ≃ 1.9, independent of the primordial spectral shape and consistent with observational data. After computing the cluster biasing parameter, bcl, we estimate the quantity βcl=Ω0.60/bclβcl=Ω00.6/bcl for the different models. Owing to the large observational uncertainties, βcl = 0.20 ± 0.05, this test does not discriminate strongly between the different models. The scale-independence of βcl, and thus ofbcl, does, however, suggest that it is probably a reliable procedure to use the linear biasing model to infer the dark matter power spectrum from observational cluster samples. We also note that the abundances of clusters predicted using the Press-Schechter theory provide strong constraints on these models: only the CHDM, LOWH and ΛCDM2 models appear to produce the correct number density of clusters.

#### Moments of the Cluster Distribution as a Test of Dark Matter Models

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis, Stefano Borgani, Lauro Moscardini, and Peter Coles
Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 441, no. 2, p. L57-L60
Publication year: 1995

## Summary

We estimate the variance and the skewness of the cluster distribution in several dark matter (DM) models. The cluster simulations are based on the Zel’dovich approximation, the low computational cost of which allows us to run 50 random realizations of each model. We compare our results with those coming from a similar analysis of a redshift sample of Abell/ACO clusters. Within the list of the considered models, we find that only the model based on Cold+Hot DM (with Ωhot=0.3) provides a good fit to the data. The standard CDM model and the low-density (Ω=0.2) CDM models, both with and without a cosmological constant term (ΩΛ=0.8) are ruled out. The tilted CDM model with primordial spectral index n=0.7 and a low Hubble constant (h=0.3) CDM model are only marginally consistent with the data.

#### The angular distribution of clusters in skewed CDM models

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Stefano Borgani, Peter Coles, Lauro Moscardini, Manolis Plionis
Monthly Notices of the Royal Astronomical Society, Volume 266, Issue 2, January 1994, Pages 524–544
Publication year: 1994

## Summary

We carry out a detailed investigation of the statistical properties of the projected (angular) distribution of galaxy clusters obtained in cold dark matter (CDM) models with both Gaussian and skewed (i.e. non-Gaussian) primordial density fluctuations. We use large numerical simulations of these skewed CDM models to construct a set of simulations of the Lick catalogue. An objective cluster-finding algorithm is used to identify regions where the projected number density of galaxies in the catalogues exceeds some density threshold criterion. In this way we construct catalogues containing the angular positions and richnesses of real and simulated clusters which are suitable for statistical analysis. For Gaussian models, the overall number of clusters is too small in the standard CDM case compared to observations, but a model with higher normalization is in much better agreement; non-Gaussian models with negative initial skewness also fit the observed numbers fairly well. We compute the angular correlation function of clusters of different richness and find a strong dependence of the clustering amplitude on richness in all models. Even with a higher normalization, the Gaussian CDM model fails to produce sufficient large-scale cluster clustering. We also find that the Lick data are better reproduced only by a CDM model with negative initial skewness; initially skew-positive models fail to produce enough large-scale clustering. This conclusion is confirmed by two other statistical analyses; the properties of the minimal spanning tree and the multifractal scaling of the real clusters are much better reproduced by skew-negative CDM models. In particular, the small-scale self-similarity in the distribution of the richest real clusters turns out to be a crucial test, which is only passed by skew-negative models. We show that a skewness–variance relation of hierarchical type is followed by skew-negative models, as well as by the more evolved Gaussian model. The results for clusters suggest that the adoption of skewed primordial fluctuations is one way to enhance the production of large-scale structure in the CDM model. The skewed models seem to have some problems in reproducing the galaxy–galaxy two-point correlation function, but these problems may be connected with the limited spatial resolution of our simulations.

#### Position Angles and Alignments of Clusters of Galaxies

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis
Astrophysical Journal Supplement Series (ISSN 0067-0049), vol. 95, no. 2, p. 401-412
Publication year: 1994

## Summary

The position angles of a large number of Abell and Shectman clusters, identified in the Lick map as surface galaxy-density enhancements, are estimated. I determine the major axis orientation of a total of 637 clusters, of which 448 are Shectman clusters (202 of which are also Abell clusters) and 189 are Abell clusters not originally detected by Shectman due to his adopted density threshold. Using published redshifts for 277 of these clusters I have detected strong nearest neighbor alignments over scales up to ~15 h^-1^ Mpc at a ~> 2.5 – 3σ significance level, while quite weak alignments are detected even up to ~60 h^-1^ Mpc. A more significant alignment signal (~4σ) is detected among all neighbors residing in superclusters and having separations <~10 h^-1^ Mpc. Again, weaker but significant alignments are found when larger separations are considered. Since my cluster sample is neither volume limited nor redshift complete (which would have tended to wash out any real alignment signal), the alignments detected should reflect a real and possibly a stronger underline effect.

#### Topology in two dimensions. IV - CDM models with non-Gaussian initial conditions

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Peter Coles, Lauro Moscardini, Manolis Plionis, Francesco Lucchin, Sabino Matarrese, Antonio Messina
Monthly Notices of the Royal Astronomical Society, Volume 260, Issue 3, February 1993, Pages 572–588
Publication year: 1993

## Summary

We use the results of N-body simulations on gravitational clustering in CDM models with both Gaussian and non-Gaussian initial conditions to generate projected galaxy catalogues with the same selection criteria as the Shane–Wirtanen counts of galaxies. We apply the technique for measuring 2D topology (the EP characteristic) discussed in the previous papers of this series to compare the statistical nature of the projected galaxy clustering in these simulated data sets with that of the observed galaxy catalogue. We find that all our models produce a topology dominated by a meatball shift when normalized to the known small-scale clustering properties of galaxies. This makes it rather difficult to separate the effects of non-linear gravitation clustering from phase correlations intrinsic to the primordial perturbations. It is, however, possible to discern differences in the topological behaviour of the same models at different stages of gravitational evolution using the projected catalogues.

Models characterized by a positive skewness of the distribution of primordial density perturbations are inconsistent with the Lick data, suggesting problems in reconciling models based on cosmic textures with observations. Gaussian CDM models fit the distribution of cell-counts only if they have a rather high normalization (b ≃ 1), but possess too low a coherence length compared with the Lick counts. This suggests that a CDM model with extra large-scale power would probably fit the available data. On the other hand, a CDM model with negative skewness of the primordial density field (chi-squared model) fits the cell counts well and matches both the shape and amplitude of the EP curve so that such models are also viable. These conclusions are shown to be rather insensitive to the form of the luminosity function used to construct the catalogues.

#### The QDOT and cluster dipoles - Evidence for a low-Omega(0) universe?

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis, Peter Coles, Paolo Catelan
Monthly Notices of the Royal Astronomical Society, Volume 262, Issue 2, May 1993, Pages 465–474
Publication year: 1993

## Summary

We have reanalysed the QDOT survey in order to investigate the convergence properties of the estimated dipole and the consequent reliability of the derived value of Ω0.60/bΩ00.6/b. We find that there is no compelling evidence that the QDOT dipole has converged within the limits of reliable determination and completeness. The value of Q0 derived by Rowan-Robinson et al. (1990) should therefore be considered only as an upper limit. Furthermore, we find strong evidence that the shell between 140 and 160 h-1 Mpc does contribute significantly to the total dipole anisotropy and therefore to the motion of the Local Group with respect to the cosmic microwave background. This shell contains the Shapley concentration, but we argue that this concentration itself cannot explain all the gravitational acceleration produced by it; there must exist a coherent anisotropy which includes this structure, but extends greatly beyond it. With the QDOT data alone, we cannot determine precisely the magnitude of any such anisotropy, but any contribution to the Local Group motion from large scales would favour a value of Ω0.60/bIRAS0.6,Ω00.6/bIRAS≤0.6, smaller than previous estimates based on IRAS galaxies; such a result would be consistent with the dipole measured from samples of rich clusters, which are much more complete at large depths.

#### Multifractal Analysis of Cluster Distribution in Two Dimensions

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Stefano Borgani, Manolis Plionis, and Riccardo Valdarnini
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 404, no. 1, p. 21-37
Publication year: 1993

## Summary

We perform a multifractal analysis on samples of galaxy clusters, obtained by cutting the Lick catalog at different values of the galaxy surface density, as well as on the Lick galaxy distribution. This kind of analysis is particularly relevant in order to properly test the scaling behavior of the cluster distribution and how it changes when peaks of different height are selected. We find that the highest selected peaks possess well-defined scaling properties up to the angular scale θ~6° – 7°, corresponding to ~20 h^-1^ Mpc at the characteristic depth of our samples. remarkably similar to the correlation length of rich galaxy systems. This finding suggests a relation between scale-invariance of the cluster distribution and nonlinear clustering. The resulting spectrum of multifractal dimensions D_q_ indicates that the rich cluster distribution is essentially space- filling in the underdense regions (D_q_~ 2 for q <~ 1), while the overdense regions (corresponding to q ~> 1) are characterized by a dimension D_q_~ 1. On the contrary, the distributions of clusters corresponding to lower peaks show no definite scaling properties. In fact, although the dimensionality of these distributions is always near to one at sufficiently small angular scales, they have a dimension around two at larger scales θ ~> 7° (thus indicating homogeneity) without evidence of scale range where the dimensions take a constant value. A similar result holds also for the distribution of Lick galaxies, which reaches homogeneity already at the scale θ~ 2.5°. We suggest that the absence of scaling for the distributions of clusters identified as lower peaks and of galaxies could well be due to projection effects that homogenize the three-dimensional clustering and, thus, are more severe in distorting the spatial scaling properties of less clustered structures. Our results indicate that well-defined scaling properties are associated in our projected samples only with the nonlinear clustering of high peaks and a corresponding dimension D ~ 1 is preferred. In order to properly test the effects of projection on the sky and of luminosity selection, we generate a three-dimensional fractal structure with an a priori known multifractal spectrum. We then assign luminosities to the points of this set drawn from a Schechter-like luminosity function, and then project onto a sphere, to generate apparent magnitude limited samples. We find that the resulting multifractal spectrum for these synthetic angular samples turns out to be distorted with respect to the original (three-dimensional) one. In particular, overdense regions are smoothed by projection, thus increasing D_q_ for q > 2, while underdense regions are filled, thus lowering the values of the negative-order dimensions. Despite this variation of the multifractal dimensions, the three-dimensional scale-invariance is however preserved. This result suggests that the break seen in D_q_(θ) (for high values of q) of the highest peak distribution is real and not due to projection or luminosity selection effects. Thus, the presence of a characteristic scale in the angular distribution of rich clusters strongly argues against a pure fractal picture extending with the same dimension over all the considered scales.

#### Large-Scale Angular Correlations in Cold Dark Matter Models

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Moscardini Lauro, Borgani Stefano, Coles Peter, Lucchin Francesco, Matarrese Sabino, Messina Antonio, Plionis Manolis
Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 413, no. 2, p. L55-L58
Publication year: 1993

## Summary

We investigate the behavior of the angular galaxy function, ω(θ, in the framework of cold dark matter (CDM) models. We compare the situation for the standard CDM model with Gaussian initial fluctuations with comparable CDM models with nonrandom phases. We have generated artificial Lick maps using the results of N-body simulations. We compare the {omega](θ) measured from the simulations with the APM correlation (scaled to the depth of the Lick map). For the Gaussian CDM model, we find that neither the standard normalization (b = 1.5) nor a more evolved model (b = 1, as suggested by the COBE data), can reproduce the correlations on large angular scales (θ >~ 2.5°). We come to a similar conclusion about CDM models with positively skewed initial fluctuation distributions. In contrast, models with initially negatively skewed fluctuations produce a ω(θ) that declines much more gently on large scales. Such models are therefore, at least in principle, capable of reconciling the lack of large-scale power of the CDM spectrum with the observed clustering of APM galaxies.

#### Topology in two dimensions – II. The Abell and ACO cluster catalogues

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis, Riccardo Valdarnini, Peter Coles
Monthly Notices of the Royal Astronomical Society, Volume 258, Issue 1, September 1992, Pages 114–124
Publication year: 1992

## Summary

We apply a method for quantifying the topology of projected galaxy clustering to the Abell and ACO catalogues of rich clusters. We use numerical simulations to quantify the statistical bias involved in using high peaks to define the large-scale structure, and we use the results obtained to correct our observational determinations for this known selection effect and also for possible errors introduced by boundary effects. We find that the Abell cluster sample is consistent with clusters being identified with high peaks of a Gaussian random field, but that the ACO shows a slight meatball shift away from the Gaussian behavior over and above that expected purely from the high-peak selection. This effect is of low statistical significance but seems consistent with other quite separate statistical analyses of the ACO sample. The most conservative explanation of this effect is that it is caused by some artefact of the procedure used to select the clusters in the two samples. On the other hand, our results could be indicating that there is either a large-scale variation in the cluster distribution due to superclustering on scales > 100 h−1 Mpc, or that there is some intrinsic non-Gaussian signature in the cluster distribution which is masked in the Abell samples by projection contamination.

#### The Angular Three-Point Function of Galaxy Clusters

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Stefano Borgani, Yi-Peng Jing, Manolis Plionis
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 395, no. 2, Aug. 20, 1992, p. 339-346
Publication year: 1992

## Summary

We estimate the three-point correlation function for samples of galaxy clusters identified from the Lick galaxy catalog at different density thresholds in order to test the ability of the biasing prescription to account for the different correlation properties of different hierarchy cosmic structures. We use two methods: the moment method and the direct counting of triplets. Sampling errors are estimated by means of bootstrap procedure. By assuming the hierarchical expression for the connected three-point function, we find that the two methods give consistent estimates of the Q-values, while the associated uncertainties are rather different. In particular, larger errors are always obtained from the moment method. In all the cases, however, our bootstrap resampling errors turn out to be greater than those obtained by other authors, based on Poissonian statistics. For clusters identified at the higher threshold levels of the underlying galaxy field we obtain Q ~ 0.5-0.8. Note, however, that even at the highest overdensity level our clusters are on average poorer than the Abell ones, and this could well be the reason why our Q-values are slightly smaller than those obtained from the Abell catalog. A general trend is found for the Q-values to take smaller values as the height of the threshold decreases. This agrees with the expectation of biasing models, where non-Gaussianity arises as a threshold effect.

#### Galaxy clusters as biased tracers of the galaxy distribution

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis, Stefano Borgani
Monthly Notices of the Royal Astronomical Society, Volume 254, Issue 2, January 1992, Pages 306–314
Publication year: 1992

## Summary

We investigate the correlation properties of different samples of galaxy clusters, which are obtained by cutting the Lick catalogue at different values of the galaxy surface density. This kind of cluster-finding algorithm is equivalent to the application of the biasing prescription to the Lick galaxy distribution. For all the samples, we find that the angular two-point correlation function can be well approximated by a power law, w(ϑ0/ϑ)1γw(ϑ0/ϑ)1−γ, with the same logarithmic slope γ2γ≃2, in the angular range 0.5ϑ100.∘5≲ϑ≲10∘. In addition, the correlation amplitude turns out to depend on the value of the limiting threshold, used to identify clusters, in exactly the same way as predicted by the biased model of galaxy formation. We also find that such a relation is reflected in a well-defined dependence of the clustering length, ϑ0ϑ0 to the cluster richness which confirms previous results obtained in the literature from the analysis of the distribution of Abell clusters.

#### Clustering of Galaxy Clusters. II. Rare Events in the Cluster Distribution

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis, Riccardo Valdarnini, and Yi-Peng Jing
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 398, no. 1, p. 12-32
Publication year: 1992

## Summary

We analyze subsamples of the Abell and ACO cluster catalogs, which are nearly complete, in order to study the large-scale structure traced by rich clusters. We use a variety of statistical techniques, minimal spanning trees, percolation void probability functions, and cluster alignments, and we compare our findings with that expected from an ensemble of simulated cluster catalogs having the same selection functions and low-order clustering as the real data. The simulations have been built applying the Zel’dovich algorithm to a δ_K_ spectrum Gaussian distributed with zero mean and random phases. The power spectrum is chosen so that the perturbed particle distribution has the same two- point correlation function as the real clusters. In this way we can test the reproducibility of the real data statistics in our simulations. We find that the minimal spanning tree distribution reveals features which are not reproduced in our simulations at a >~ 3 σ level. Similarly, not reproduced are the shapes of many superclusters, found by a friends of-friends algorithm. These discrepancies are not alleviated when increasing the clustering strength or the large scale power in the simulations. We suggest that the real cluster distribution might be described by a statistic more complicated than a simple Gaussian.

#### Clustering of Galaxy Clusters. I. Is the Spatial Cluster-Cluster Correlation Function Enhanced Significantly by Contaminations?

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Yi-Peng Jing, Manolis Plionis, and Riccardo Valdarnini
Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 389, April 20, 1992, p. 499-509
Publication year: 1992

## Summary

We suggest that a consistency check of the redshift correlation function between three redshift samples of Abell clusters is able to distinguish whether the positive redshift correlations, found by Sutherland (hereafter Sutherland effect), at large redshift and small angular separations are mainly the result of foreground/background cluster contaminations or of real clustering. Our results do not support that contaminations are the origin of this effect. Instead, the observed features of the redshift correlation function are compatible with strong, richness-dependent clustering of small numbers of clusters which often results in supercluster elongations along the line of sight. To test this suggestion we have constructed a set of simulated cluster catalogs which have the same two-point correlation function and the same selection effects as the real samples. We found that the Sutherland effect is quite common in the simulated catalogs, which are completely free of contamination effects. The X^2^ test shows that the observed redshift correlation functions can be reproduced in the simulations more than 15%- 30% of the time, thus supporting that contaminations of the Abell cluster catalog are not necessarily the cause of this effect. Our results support that the rich cluster-cluster correlation function, as estimated by Bahcall & Soneira, is not significantly enhanced by such effects, and therefore the biased CDM model is still in disagreement with the observed clustering of clusters.

#### Projected and intrinsic shapes of galaxy clusters

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis, John D. Barrow, Carlos S. Frenk
Monthly Notices of the Royal Astronomical Society, Volume 249, Issue 4, April 1991, Pages 662–677
Publication year: 1991

## Summary

We identify a large number of galaxy clusters in the Lick map using an algorithm based on an overdensity criterion. The resulting catalogues contain ~ 6000 clusters (with|b|40)(with|b|≥40∘) out of which 753 are Abell clusters. We determine ellipticities and major axis orientations for a suitable subset of this sample, including 397 Abell clusters. We find that the distribution of projected axial ratios is approximately Gaussian with a mean of ~ 0.6 and a standard deviation of ~ 0.2. We investigate methods to invert the distribution of apparent axial ratios in order to obtain the distribution of intrinsic axial ratios. We find that the apparent shapes of clusters in our sample are inconsistent with those expected from a population of oblate spheroids seen in projection. The observed distribution could come from a population of prolate spheroids with a Gaussian distribution of axial ratios with mean ~ 0.5 and standard deviation ~ 0.15. However, we show that in the general case where objects are triaxial or there is a mixture of oblate and prolate configurations, the inversion to intrinsic shapes is not necessarily unique. There appears to be a significant increase of ellipticity with redshift out to the limit of our sample, z ~ 0.15. This could be due to a distance dependent contamination by projection effects or to intrinsic evolution in the cluster population.

#### Evidence for large-scale structure on scales about ∼300h−1 Mpc

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis, Riccardo Valdarnini
Monthly Notices of the Royal Astronomical Society, Volume 249, Issue 1, March 1991, Pages 46–61
Publication year: 1991

## Summary

We have analysed the combined Abell–ACO cluster catalogue with m1016.4m10≲16.4 which is 80 per cent redshift complete. Taking into account different models of the extinction region we find the cluster dipole pointing within 10≲10∘ of the CMB dipole direction which indicates that clusters are tracers of mass. The dipole amplitude converges at 150h1∼150h−1 Mpc, within the completeness limit of the catalogue. We find that the anisotropy which is responsible for the LG motion with respect to the CMB rest frame extends in depth and has a scalelength of 300h1∼300h−1 Mpc. Most of the Local Group peculiar velocity is induced by matter fluctuations within 50h1∼50h−1 Mpc. Shapley’s concentration (otherwise known as ‘Giant Attractor’ or ‘alpha region’) at a depth of 140h1∼140h−1 Mpc, is responsible for ≲ 20 per cent of the Local Group’s peculiar velocity. Finally, using linear perturbation theory, we find that Ω00.3

#### On the determination of dipoles from incomplete galaxy catalogues

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis
Monthly Notices of the Royal Astronomical Society, Volume 238, Issue 2, May 1989, Pages 417–425
Publication year: 1989

## Summary

The spherical harmonic and random filling methods, used to account for incomplete sky coverage in dipole calculations, are investigated. A study of their comparative performances is carried out and the former method is shown to be more accurate. Using both these methods, an attempt is made to find the regions responsible for the dipole in the Lahav optical catalogue. We find, in accordance with Lahav et al., that the Virgo–Centaurus region provides most of the dipole power. We also find strong indications that these structures are embedded within a global underlying anisotropy which could account for the dipole even if the Virgo–Centaurus region were absent and the anisotropics in the rest of the sphere, as quantified by the expansion of the galaxy surface density to quadrupole order, were extrapolated to this region. A picture of a global anisotropy in the distribution of galaxies throughout the unit sphere on which the various ‘attractors’ are superimposed is favoured.

#### Large-scale optical dipole anisotropy

Δημοσιεύσεις σε Έγκυρα Διεθνή Περιοδικά με Κριτές
Manolis Plionis
Monthly Notices of the Royal Astronomical Society, Volume 234, Issue 2, September 1988, Pages 401–416
Publication year: 1988

## Summary

Large-scale number-count anisotropies are present in the Shane & Wirtanen galaxy catalogue. These anisotropies add up to a dipole pointing within 9°−34° of the microwave background radiation dipole, consistent with the dipoles of a shallower optical and of the IRAS galaxy catalogues. The dipole amplitude is ∼4.7±1.3 per cent. The high surface density regions show a dipole anisotropy pointing roughly towards the direction of the recently reported large-scale streaming motion of a sample of elliptical galaxies, while the low density regions appear to have a dipole distribution pointing roughly towards the Local Group motion with respect to the frame defined by these elliptical galaxies. These anisotropies appear to be caused neither by systematic and random errors nor by galactic or atmospheric absorption.