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 \rho _ { HI } of high redshift , H i ‘ disks ’ 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 to 5 ( 738 DLAs ) , f ( N _ { HI } ,X ) is well fitted by a double power-law with a break column density N _ { d } = 10 ^ { 21.55 \pm 0.04 } { cm ^ { -2 } } and low/high-end exponents \alpha = -2.00 \pm 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 21cm 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 two during the \approx 2 Gyr interval from z = 4 to 2.2 giving corresponding decreases in both the total covering fraction and \rho _ { HI } .
At z \approx 2 , these quantities match the present-day values suggesting no evolution during the past \approx 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 \approx 3 establishing the antecedants of current early-type galaxies .
The perceived constancy of \rho _ { HI } , meanwhile , implies that H i 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 .