DAMPED Ly α SURVEY : DATA RELEASE 3

We present the results from a damped Lyα survey of the Sloan Digital Sky Survey, Data Release 3. We have discovered over 500 new damped Lyα systems at z > 2.2 and the complete statistical sample for z > 1.6 has more than 600 damped Lyα galaxies. We measure the H I column density distribution fHI(N,X) and its zeroth and first moments (the incidence lDLA and gas mass-density Ω DLA g of damped Lyα systems, respectively) as a function of redshift. The key results include: (1) the full SDSS-DR3 fHI(N,X) distribution (z ∼ 3.06) is well fit by a Γ-function (or double power-law) with ‘break’ column density Nγ = 10 21.5±0.1 cm and ‘faint-end’ slope α = −1.8 ± 0.1; (2) the shape of the fHI(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 ≈ 1Gyr between the redshift interval z = [3., 3.5] to z = [2.2, 2.5]; and (4) the incidence and gas mass density of damped Lyα 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 Lyα analysis and identify only one important effect: we measure 40± 20% higher Ω 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 ΛCDM, we find all of the models significantly underpredict lDLA at z = 3 and only SPH models with significant feedback (Nagamine et al.) may reproduce Ω g at high redshift. Based on our results for the damped Lyα systems, we argue that the Lyman limit systems contribute ≈ 33% of the universe’s H I atoms at all redshifts z = 2 to 5. Furthermore, we infer that the fHI(N,X) distribution for NHI < 10 20 cm has an inflection with slope d log f/d logN > −1. We advocate a new mass density definition – the mass density of predominantly neutral gas Ω g – to be contrasted with the mass density of gas associated with H I atoms. We contend the damped Lyα systems contribute > 80% of Ω g at all redshifts and therefore are the main reservoirs for star formation. Subject headings: galaxies: evolution — intergalactic medium — quasars: absorption lines