Damped Lyα systems at z < 1.65:: The expanded Sloan Digital Sky Survey Hubble Space Telescope sample

We present results of our Hubble Space Telescope Cycle 11 survey for low-redshift (z < 1.65) damped Ly alpha (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-FeII systems that are known signatures of high column density neutral gas. In total, including our previous surveys, UV observations of Ly alpha absorption in 197 Mg II systems with z < 1.65 and rest equivalent width (REW) W-0(lambda 2796) >= 0.3 angstrom have now been obtained. The main results are as follows: (1) The success rate of identifying DLAs in a Mg II sample with Wk(0)(lambda 2796) >= 0.5 angstrom and Fe II W-0(lambda 2600) >= 0.5 angstrom is 36% +/- 6% and increases to 42% +/- 7% for systems with W-0(lambda 2796)/W-0(lambda 2600) < 2 and Mg I W-0(lambda 2852) > 0.1 angstrom. (2) The mean H I column density of Mg II systems with 0.3 angstrom <= W-0(lambda 2796) < 0.6 angstrom is < N(H I)> = (9.7 +/- 2.2); 10(18) cm(-2). For the larger REW systems in our sample, < N(H I)> = (3.5 +/- 0.7) x 10(20) cm(-2). (3) The DLA incidence per unit redshift for 0 < z < 5 is n(DLA)(z) = n(0)(1 + z)(gamma), where n(0) = 0.044 +/- 0.005 and gamma = 1.27 +/- 0.11. This parameterization is consistent with no evolution for z less than or similar to 2 (Omega(Lambda) = 0.7, Omega(M) = 0.3) but exhibits significant evolution for z greater than or similar to 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, Omega(DLA) approximate to 1 x 10(-3). This is larger than Omega(gas)(z = 0) by a factor of approximate to 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 Omega(DLA) from z = 0.5 to 0.