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  ( N _{H  i} < 10¹⁵ cm⁻²)  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   r _0,gg= 3.1 ± 0.1 Mpc  h ⁻¹  and   r _0,ag= 1.4 ± 0.1 Mpc  h ⁻¹  , respectively), in contrast with the recent results of Ryan-Weber, 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 Ryan-Weber. 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 data sets is ∼1σ only.