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.