On the (Non)evolution of H I Gas in Galaxies Over Cosmic Time

We present new results on the frequency distribution of projected HI column densities f (N, X), total comoving covering fraction, and integrated mass densities rhoHI of high redshift, HI `disks' from a survey of damped Lya systems (DLAs) in the Sloan Digital Sky Survey, Data Release 5. For the full sample spanning z=2. 2 to 5 738 DLAs, f (N, X) is well fitted by a double power-law with a break column density Nd = 10^ (21. 55 +/- 0. 04) and low/high-end exponents alpha = -2. 00 +/- 0. 05, -6. 4^+1. 1-₁. ₆. The shape of f (N, X) is invariant during this redshift interval and also follows the projected surface density distribution of present-day HI disks as inferred from 21cm observations. We conclude that HI gas has been distributed in a self-similar fashion for the past 12Gyr. The normalization of f (N, X), in contrast, decreases by a factor of two during the ~2Gyr interval from z=4 to 2. 2 giving corresponding decreases in both the total covering fraction and rhoHI. At z~2, these quantities match the present-day values suggesting no evolution during the past ~10Gyr. 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 antecedants of current early-type galaxies. The perceived constancy of rhoHI, meanwhile, implies that HI gas is a necessary but insufficient pre-condition for star formation and that the global star-formation rate is driven by the accretion and condensation of fresh gas from the intergalactic medium.