Predictions for the relation between strong H I 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 greater than or similar to 10(17) cm(-2)) and galaxies at redshift z = 3. We find a strong anticorrelation 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 a systems (DLAs) are similar to 10 and similar to 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 similar to 1 and similar to 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-circle dot and star formation rate < 10(-1) M-circle dot yr(-1). 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 greater than or similar to 10(9) M-circle dot. 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typically far too faint to be detectable with current instrumentation, which is consistent with the high rate of (often unpublished) non-detections in observational searches for the galaxy counterparts of strong HI absorbers.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.