The HI Content of the Universe over the Past 10 Gyrs

TL;DR

This study uses HST UV quasar spectra to identify low-redshift damped Ly-alpha absorbers, estimating the evolution of the universe's neutral hydrogen content over the last 10 billion years.

Contribution

First blind survey of DLAs at z < 1.6 using HST UV spectra, providing new measurements of neutral hydrogen density evolution.

Findings

Lower incidence rate of DLAs at z ~ 0.6 compared to earlier estimates.

HI mass density at z ~ 0.6 is significantly lower than at higher redshifts.

Results suggest a decreasing trend in neutral gas density over 10 Gyrs.

Abstract

We use the Hubble Space Telescope (HST) archive of ultraviolet (UV) quasar spectroscopy to conduct the first blind survey for damped Ly-alpha absorbers (DLAs) at low redshift (z < 1.6). Our statistical sample includes 463 quasars with spectral coverage spanning a total redshift path, dz = 123.3 or an absorption path, dX = 229.7. Within this survey path, we identify 4 DLAs, defined as absorbers with HI column density N(HI) >= 10^20.3cm-2, which implies an incidence per absorption length, l(X)= 0.017(+0.014-0.008) at a median survey path redshift of z=0.623. While our estimate of l(X) is lower than earlier estimates at z ~ 0 from HI 21cm emission studies, the results are consistent within the measurement uncertainties. Our dataset is too small to properly sample the N(HI) frequency distribution function f(N(HI),X), but the observed distribution agrees with previous estimates at z > 2. Adopting the z > 2 shape of f(N(HI),X), we infer an HI mass density at z ~ 0.6 of rho_HI = 0.25(+0.20-0.12) x 10^8 Msol Mpc-3. This is significantly lower than previous estimates from targeted DLA surveys with the HST, but consistent with results from low-z HI 21cm observations, and suggests that the neutral gas density of the universe has been decreasing over the past 10 Gyrs.