THE H i CONTENT OF THE UNIVERSE OVER THE PAST 10 GYR

Abstract

We use the Hubble Space Telescope ( HST ) archive of ultraviolet (UV) quasar spectroscopy to conduct the first blind survey for damped Ly α absorbers (DLAs) at low redshift ( ). Our statistical sample includes 463 quasars with spectral coverage spanning a total redshift path or an absorption path . Within this survey path, we identify 4 DLAs defined as absorbers with H i column density cm −2 , which implies an incidence per absorption length at a median survey path redshift of z  = 0.623. While our estimate of is lower than earlier estimates at from H i  21 cm emission studies, the results are consistent within the measurement uncertainties. Our data set is too small to properly sample the frequency distribution function , but the observed distribution agrees with previous estimates at . Adopting the shape of , we infer an H i mass density at of . This is significantly lower than previous estimates from targeted DLA surveys with the HST , but consistent with results from low- z H i  21 cm observations, and suggests that the neutral gas density of the universe has been decreasing over the past 10 Gyr.