2006ApJ...636..610R

We present results of our Hubble Space Telescope Cycle 11 survey for low-redshift (z<1.65) damped Lyα (DLA) systems in the UV spectra of quasars selected from the Sloan Digital Sky Survey (SDSS) Early Data Release. These quasars have strong intervening Mg II-Fe II systems that are known signatures of high column density neutral gas. In total, including our previous surveys, UV observations of Lyα absorption in 197 Mg II systems with z<1.65 and rest equivalent width (REW) W^λ2796₀≥0.3 Å have now been obtained. The main results are as follows: (1) The success rate of identifying DLAs in a Mg II sample with W^λ2796₀≥0.5 Å and Fe II W^λ2600₀≥0.5 Å is 36%±6% and increases to 42%±7% for systems with W^λ2796₀/W^λ2600₀<2 and Mg I W^λ2852₀>0.1 Å. (2) The mean H I column density of Mg II systems with 0.3 Å≤W^λ2796₀<0.6 Å is N(H I)=(9.7±2.2)x10¹⁸/cm2. For the larger REW systems in our sample, N(H I)=(3.5±0.7)x10²⁰/cm2. (3) The DLA incidence per unit redshift for 0<z<5 is n_DLA(z)=n₀(1+z)^γ, where n₀=0.044±0.005 and γ=1.27±0.11. This parameterization is consistent with no evolution for z≲2 (Ω_Λ=0.7, Ω_M=0.3) but exhibits significant evolution for z≳2. (4) The cosmological neutral gas mass density due to DLAs is constant in the redshift interval 0.5<z<5.0 to within the uncertainties, Ω_DLA~1x10^–3. This is larger than Ω_gas(z=0) by a factor of ~2. (5) The slope of the H I column density distribution does not change significantly with redshift. However, the low-redshift distribution is marginally flatter due to the higher fraction of high column density systems in our sample. (6) Finally, using the precision of Mg II survey statistics, we show that under the assumption of constant DLA fraction and H I column density suggested by our current sample, there may be evidence of a decreasing Ω_DLAfrom z=0.5 to 0.