ON THE (NON)EVOLUTION OF H I GAS IN GALAXIES OVER COSMIC TIME

We present new results on the frequency distribution of projected H I column densities f(N HI, X), total comoving covering fraction, and integrated mass densities ρHI of high-redshift, H I galactic gas from a survey of damped Lyα systems (DLAs) in the Sloan Digital Sky Survey, Data Release 5. For the full sample spanning z = 2.2-5 (738 DLAs), f(N HI, X) is well fitted by a double power law with a break column density Nd = 1021.55±0.04 cm-2 and low/high-end exponents α = -2.00 ± 0.05, - 6.4+1.1-1.6. The shape of f(N HI, X) is invariant during this redshift interval and also follows the projected surface density distribution of present-day H I disks as inferred from 21 cm observations. We conclude that H I gas has been distributed in a self-similar fashion for the past 12 Gyr. The normalization of f(N HI, X), in contrast, decreases by a factor of 2 during the ∼2 Gyr interval from z = 4-2.2 with coincident decreases in both the total covering fraction and ρHI. At z ∼ 2, these quantities match the present-day values suggesting no evolution during the past ∼10 Gyr. We argue that the evolution at early times is driven by 'violent' processes that removes gas from nearly half the galaxies at z ∼ 3 establishing the antecedents of current early-type galaxies. The perceived constancy of ρHI, meanwhile, implies that H I gas is a necessary but insufficient precondition for star formation and that the global star formation rate is driven by the accretion and condensation of fresh gas from the intergalactic medium.