Surveys for z > 3 damped Lyα absorption systems:: The evolution of neutral gas

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 Ly alpha 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 Ly alpha absorbers with column densities N-Ht greater than or equal to 2 x 10(20) atoms cm(-2) covering the redshift range 0.008 less than or equal to z less than or equal to 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 greater than or equal to 10(21) atoms cm(-2)) and more lower column density systems (N-HI = 2-4 x 10(20) atoms cm(-2)). 2. The frequency of very high column density absorbers (N-HI greater than or equal to 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 less than or equal to z less than or equal to 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 log N-HI greater than or equal to 21 have yet been detected at z > 4, although we have increased the redshift path surveyed by approximate to 60%. 3. With our current data set, the comoving mass density of neutral gas, Omega (g), appears to peak at 3.0 < z < 3.5, but the uncertainties are still too large to determine the precise shape of Omega (g). The statistics are consistent with a constant value of Omega (g), for 2 < z < 4. There is still tentative evidence for a drop-off at,- > 4, as indicated by earlier data sets. If we define R-g* drop Omega (g)/Omega (*), where R-g is the ratio of the peak value of Omega (g) to Omega (*), the mass density in galaxies in the local universe, we find values of R-g* = 0.25-0.5 at,- - 3, depending on the cosmology. For an Omega = 1 universe with a zero cosmological constant, R-g* = 0.25-0.5. For an Omega = 1 universe with a positive cosmological constant (Omega (Lambda) = 0.7, Omega (M) = 0.3), we find R-g* approximate to 0.25. For a universe with Omega (Lambda) = 0 and Omega (M) = 0.3, we find R-g* approximate to 0.3. 4. Omega (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 3 < 1.5 that suggest that (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.