Predictions for the relation between strong HI absorbers and galaxies at redshift 3

We combine cosmological, hydrodynamical simulations with accurate radiative transfer corrections to investigate the relation between strong HI absorbers (N_HI >~ 10^17 /cm^2) and galaxies at redshift z = 3. We find a strong anti-correlation between the column density and the impact parameter that connects the absorber to the nearest galaxy. The median impact parameters for Lyman Limit (LL) and Damped Lyman-α (DLA) systems are ~10 and ~1 proper kpc, respectively. If normalized to the size of the halo of the nearest central galaxy, the median impact parameters for LL and DLA systems become ~1 and ~10^-1 virial radii, respectively. At a given HI column density, the impact parameter increases with the mass of the closest galaxy, in agreement with observations. We predict most strong HI absorbers to be most closely associated with extremely low-mass galaxies, M_star < 10^8 M_sun and star formation rate <10^-1 M_sun/yr. We also find a correlation between the column density of absorbers and the mass of the nearest galaxy. This correlation is most pronounced for DLAs with N_HI > 10^21 /cm^2 which are typically close to galaxies with M_star >~ 10^9 M_sun. Similar correlations exist between column density and other properties of the associated galaxies such as their star formation rates, halo masses and HI content. The galaxies nearest to HI absorbers are typically far too faint to be detectable with current instrumentation, which is consistent with the high rate of (often unpublished) non-detections in observational searches for the galaxy counterparts of strong HI absorbers. Moreover, we predict that the detected nearby galaxies are typically not the galaxies that are most closely associated with the absorbers, thus causing the impact parameters, star formation rates and stellar masses of the observed counterparts to be biased high.