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.