THROUGH THICK AND THIN-H I ABSORPTION IN COSMOLOGICAL SIMULATIONS

We investigate the column density distribution function of neutral hydrogen at redshift z = 3 using a cosmological simulation of galaxy formation from the OverWhelmingly Large Simulations project. The base simulation includes gravity, hydrodynamics, star formation, supernovae feedback, stellar winds, chemodynamics, and element-by-element cooling in the presence of a uniform UV background. Self-shielding and formation of molecular hydrogen are treated in post-processing, without introducing any free parameters, using an accurate reverse ray-tracing algorithm and an empirical relation between gas pressure and molecular mass fraction. The simulation reproduces the observed z = 3 abundance of Ly alpha forest, Lyman limit, and damped Ly alpha H I absorption systems probed by quasar sight lines over 10 orders of magnitude in column density. Self-shielding flattens the column density distribution for N-HI > 1018 cm(-2), while the transition to fully neutral gas and conversion of H I to H-2 steepen it around column densities of N-H (I) = 10(20.3) cm(-2) and N-H (I) = 10(21.5) cm(-2), respectively.