Damped Lyα Systems atz < 1.65: The Expanded Sloan Digital Sky SurveyHubble Space TelescopeSample

Abstract

We present results of our Hubble Space Telescope Cycle 11 survey for low-redshift (z 0.1 ?. (2) The mean H I column density of Mg II systems with 0.3 ? ? W < 0.6 ? is = (9.7 ? 2.2) ? 1018 cm-2. For the larger REW systems in our sample, = (3.5 ? 0.7) ? 1020 cm-2. (3) The DLA incidence per unit redshift for 0 < z < 5 is nDLA(z) = n0(1 + z)?, where n0 = 0.044 ? 0.005 and ? = 1.27 ? 0.11. This parameterization is consistent with no evolution for z 2 (?? = 0.7, ?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, ?DLA ? 1 ? 10-3. This is larger than ?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 ?DLA from z = 0.5 to 0.