The SDSS Damped Lyα Survey: Data Release 3

We present the results from a damped Lya survey of the Sloan Digital SkySurvey, Data Release 3 based on over 500 new damped Lya systems at z>2.2. Wemeasure the HI column density distribution f(N) and its zeroth and firstmoments (the incidence l(X) and gas mass-density O_dla of damped Lya systems,respectively) as a function of redshift. The key results include: (1) the f(N)distribution is well fit by a Gamma-function with `break' column density logN_g=10^21.5 and `faint-end' slope alpha=-1.8; (2) the shape of the f(N)distributions do not show evolution with redshift; (3) l(X) and O_dla decreaseby 35% and 50% during ~1Gyr between redshift z=[3.,3.5] to z=[2.2,2.5]; and (4)l(X) and O_dla in the lowest SDSS redshift bin (z=2.2) are consistent with thecurrent values. We investigate systematic errors in damped Lya analysis andidentify only one important effect: we measure 40 +/- 20% higher O_dla valuestoward a subset of brighter quasars than toward a faint subset. This effectruns contrary to the bias associated with dust obscuration and suggests thatgravitational lensing may be important. Comparing the results against models ofgalaxy formation, we find all of the models significantly underpredict l(X) atz=3 and only SPH models with significant feedback may reproduce O_dla at highredshift. We argue that the Lyman limit systems contribute ~1/3 of theuniverse's HI atoms at all redshifts z=2 to 5 and that the f(N) distributionfor N(HI)<10^20 has an inflection with slope >-1. We advocate a new massdensity definition -- the mass density of predominantly neutral gas O_neut --to be contrasted with the mass density of gas associated with HI atoms. Wecontend the damped Lya systems contribute >80% of O_neut at all redshifts andtherefore are the main reservoirs for star formation. [abridged]Comment: 24 pages, 22 figures. Accepted to ApJ, August 15, 200