The SDSS damped Lyα survey:: Data release 3

We present the results from a damped Ly alpha survey of the Sloan Digital Sky Survey, Data Release 3. We have discovered over 500 new damped Ly alpha systems at z > 2: 2, and the complete statistical sample for z > 1: 6 has more than 600 damped Ly alpha galaxies. We measure the H I column density distribution f(H I) (N, X) and its zeroth and first moments ( the incidence l(DLA) and gas mass density Omega(g)(DLA) of damped Ly alpha systems, respectively) as a function of redshift. The key results include: ( 1) the full SDSS DR3 f(H I) ( N, X) distribution ( z similar to 3: 06) is well fit by a Gamma function ( or double power law) with "break'' column density N-gamma 10(21.5 +/- 0.1) cm(-2) and "faint-end'' slope alpha = 1.8 +/- 0.1; ( 2) the shape of the f(H I) (N, X) distributions in a series of redshift bins does not show evolution; ( 3) the incidence and gas mass density of damped systems decrease by 35% +/- 9% and 50% +/- 10% during approximate to 1 Gyr between the redshift intervals z 3: 0; 3.5] and z = [2.2; 2.5]; and ( 4) the incidence and gas mass density of damped Ly alpha systems in the lowest SDSS redshift bin ( z 2: 2) are consistent with the current values. We investigate a number of systematic errors in damped Ly alpha analysis and identify only one important effect: we measure 40% +/- 20% higher Omega(g)(DLA) g values toward a subset of brighter quasars than toward a faint subset. This effect is contrary to the bias associated with dust obscuration and suggests that gravitational lensing may be important. Comparing the results against several models of galaxy formation in Lambda CDM,we find that all of the models significantly underpredict l(DLA) at z = 3, and only SPH models with significant feedback can reproduce Omega(g)(DLA) at high redshift. Based on our results for the damped Ly alpha systems, we argue that the Lyman limit systems contribute approximate to 33% of the universe's H I atoms at all redshifts z = 2-5. Furthermore, we infer that the f(H I) ( N, X) distribution for N-H I < 10(20) cm(-2) has an inflection with slope d log f/d log N > -1. We advocate a new mass density definition, the mass density of predominantly neutral gas Omega(g)(neut), to be contrasted with the mass density of gas associated with H I atoms. We contend the damped Ly alpha systems contribute > 80% of Omega(g)(neut) at all redshifts and therefore are the main reservoirs for star formation.