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 Lyα absorbers (DLAs) at low redshift ($z\lt 1.6$). Our statistical sample includes 463 quasars with spectral coverage spanning a total redshift path ${\rm{\Delta }}z=123.3$ or an absorption path ${\rm{\Delta }}X=229.7$. Within this survey path, we identify 4 DLAs defined as absorbers with H i column density ${N}_{{\rm{H}}{\rm{I}}}$ $\geqslant \;{10}^{20.3}$ cm−2, which implies an incidence per absorption length ${{\ell }}_{{\rm{DLA}}}(X)$ $=\;{0.017}_{-0.008}^{+0.014}$ at a median survey path redshift of z = 0.623. While our estimate of ${{\ell }}_{{\rm{DLA}}}(X)$ is lower than earlier estimates at $z\approx 0$ 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 ${N}_{{\rm{H}}{\rm{I}}}$ frequency distribution function $f({N}_{{\rm{H}}{\rm{I}}},X)$, but the observed distribution agrees with previous estimates at $z\gt 2$. Adopting the $z\gt 2$ shape of $f({N}_{{\rm{H}}{\rm{I}}},X)$, we infer an H i mass density at $z\sim 0.6$ of ${\rho }_{{\rm{H}}\;{\rm{I}}}^{{\rm{DLA}}}$ $=\;{0.25}_{-0.12}^{+0.20}\times {10}^{8}{M}_{\odot }\;{{\rm{Mpc}}}^{-3}$. 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.