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 H I absorbers ({N_{H I}}≳ 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 α systems (DLAs) 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 H I}column density, the impact parameter increases with the mass of the closest galaxy, in agreement with observations. We predict most strong H I absorbers to be most closely associated with extremely low-mass galaxies, M_⋆ < 10⁸ M_⊙ and star formation rate < 10^{- 1} M_⊙ 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_{H I}}> 10^{21} cm^{-2} which are typically close to galaxies with M_⋆ ≳ 10⁹ M_⊙. Similar correlations exist between column density and other properties of the associated galaxies such as their star formation rates, halo masses and H I content. The galaxies nearest to H I 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typically far too faint to be detectable with current instrumentation, which is consistent with the high rate of (often unpublished) non-detections in observational searches for the galaxy counterparts of strong H I} absorbers.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.