Neutral Gas Density in Damped Lyα Systems

We estimate the intrinsic neutral gas density in damped Lyα (DLA) systems (Ω) in the redshift range 2.2 ≲ z ≲ 5 from the SDSS DR3 DLA sample of optically selected quasars. We take into account self-consistently the obscuration on background quasars due to the dust present in damped Lyα systems. We model the column density and redshift distribution of these systems by using both a nonparametric and a parametric approach. Under conservative assumptions on the dust content of DLA systems, we show that selection effects lead to underestimating the intrinsic neutral gas density by at least 15% with respect to the observed neutral gas density. Over the redshift range 2.2-5.5, we find Ω = 0.97 × 10-3, where the first set of error bars gives the 1 σ random errors and the second set gives the modeling uncertainty dependent on the fraction of metals in dust, from 0% to 50%. This value compares with Ω = 0.82 × 10-3 (1 σ error bars), which is obtained when no correction for dust is introduced. In the model with half of the metals mass in dust, we cannot constraint Ω at a confidence level higher than 90%. In this case there is indeed a probability of about 10% that the intrinsic column density distribution of DLA systems is a power law f(N) ∝ 1/N. In contrast, with 25% of the metals in dust, the most realistic estimate, a power law is ruled out at a 99.5% confidence level.