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

We present new results on the frequency distribution of projected H I column densities f(N-H I, X), total comoving covering fraction, and integrated mass densities rho(H I) of high-redshift, H I galactic gas from a survey of damped Ly alpha systems (DLAs) in the Sloan Digital Sky Survey, Data Release 5. For the full sample spanning z = 2.2-5 ( 738 DLAs), f(N-H I, X) is well fitted by a double power law with a break column density N-d = 10(21.55 +/- 0.04) cm(-2) and low/high-end exponents alpha = -2.00 +/- 0.05, -6.4(1.6)(+1.1). The shape of f(N-H I, 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-H I, X), in contrast, decreases by a factor of 2 during the approximate to 2 Gyr interval from z = 4-2.2 with coincident decreases in both the total covering fraction and rho(H I). At z approximate to 2, these quantities match the present-day values suggesting no evolution during the past approximate to 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 approximate to 3 establishing the antecedents of current early-type galaxies. The perceived constancy of rho(H I), meanwhile, implies that Hi 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.