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 AngloAustralian 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_{HI}>=2×10^{20} 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.1. For the first time, we determine a statistically significant steepening in the column density distribution function at redshifts z>4.0 (greater than 99.7% confidence). The steepening of the distribution function is due to both fewer very high column density absorbers (N_{HI}>=10^{21} atoms cm^{2}) and more lower column density systems (N_{HI}=24×10^{20} atoms cm^{2}).2. The frequency of very high column density absorbers (N_{HI}>=10^{21} atoms cm^{2}) reaches a peak in the redshift range 1.5<z<4, when the universe is 10%30% of its present age. Although the sample size is still small, the peak epoch appears to be 3.0<=z<=3.5. The highest column density absorbers disappear rapidly toward higher redshifts in the range z=3.5>4.7 and lower redshifts z=3.0>0. None with column densities logN_{HI}>=21 have yet been detected at z>4, although we have increased the redshift path surveyed by ~60%.3. With our current data set, the comoving mass density of neutral gas, Ω_{g}, appears to peak at 3.0<z<3.5, but the uncertainties are still too large to determine the precise shape of Ω_{g}. The statistics are consistent with a constant value of Ω_{g} for 2<z<4. There is still tentative evidence for a dropoff at z>4, as indicated by earlier data sets. If we define R_{g*}≡Ω_{g}/Ω_{*}, where R_{g*} is the ratio of the peak value of Ω_{g} to Ω_{*}, the mass density in galaxies in the local universe, we find values of R_{g*}=0.250.5 at z~3, depending on the cosmology. For an Ω=1 universe with a zero cosmological constant, R_{g*}~0.5. For an Ω=1 universe with a positive cosmological constant (Ω_{Λ}=0.7, Ω_{M}=0.3), we find R_{g*}~0.25. For a universe with Ω_{Λ}=0 and Ω_{M}=0.3, we find R_{g*}~0.3.4. Ω_{g} decreases with redshift for the interval z=3.5>0.008 for our data set, but we briefly discuss new results from Rao & Turnshek for z<1.5 that suggest that Ω_{g} (z<1.5) may be higher than previously determined.To make the data in our statistical sample more readily available for comparison with scenarios from various cosmological models, we provide tables that include all 646 QSOs from our new survey and previously published surveys. They list the minimum and maximum redshift defining the redshift path along each line of sight, the QSO emission redshift, and when an absorber is detected, the absorption redshift and measured H I column density. Some of the observations presented here were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. Observations were also made using the 3.9 m AngloAustralian Telescope at the AngloAustralian Observatory and the Shane 3 m Telescope at the Lick Observatory.
 Publication:

The Astrophysical Journal
 Pub Date:
 November 2000
 DOI:
 10.1086/317138
 arXiv:
 arXiv:astroph/0006044
 Bibcode:
 2000ApJ...543..552S
 Keywords:

 Galaxies: Evolution;
 Galaxies: Intergalactic Medium;
 Galaxies: Quasars: Absorption Lines;
 Astrophysics
 EPrint:
 46 pages, including 15 figures and 9 tables, to appear in ApJ