Surveys for z > 3 Damped Lyα Absorption Systems: The Evolution of Neutral Gas

Abstract

We have completed spectroscopic observations using LRIS on the Keck 1 telescope of 30 very high redshift quasars, 11 selected for the presence of damped Lyα absorption systems and 19 with redshifts z > 3.5 not previously surveyed for absorption systems. We have surveyed an additional 10 QSOs with the Lick 120'' and the Anglo-Australian Telescope. We have combined these with previous data, resulting in a statistical sample of 646 QSOs and 85 damped Lyα absorbers with column densities N ≥ 2 × 1020 atoms cm-2 covering the redshift range 0.008 ≤ z ≤ 4.694. Four main features of how the neutral gas in the universe evolves with redshift are evident from these data.