The relation between Lyman–α absorbers and gas–rich galaxies in the local universe

We use high-resolution hydrodynamical simulations to investigate the spatial correlation between weak (NHI < 10 cm) Lyα absorbers and gas–rich galaxies in the local universe. We confirm that Lyα absorbers are preferentially expected near gas–rich galaxies and that the degree of correlation increases with the column density of the absorber. The real–space galaxy auto–correlation is stronger than the cross–correlation (correlation lengths r0,gg = 3.1 ± 0.1 Mpc h and r0,ag = 1.4 ± 0.1 Mpc h, respectively), in contrast with the recent results of (Ryan-Weber 2006, RW06), and the auto–correlation of absorbers is very weak. These results are robust to the presence of strong galactic winds in the hydrodynamical simulations. In redshift–space a further mismatch arises since at small separations the distortion pattern of the simulated galaxy-absorber cross-correlation function is different from the one measured by RW06. However, when sampling the intergalactic medium along a limited number of lines–of–sight, as in the real data, uncertainties in the cross correlation estimates are large enough to account for these discrepancies. Our analysis suggests that the statistical significance of difference between the cross–correlation and auto–correlation signal in current datasets is ∼ 1-σ only.