The relation between Lyman alpha 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) < 10(15) cm(-2)) Ly alpha absorbers and gas-rich galaxies in the local Universe. We confirm that Ly alpha 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 similar to 1 sigma only.