The HI Content of the Universe over the Past 10 Gyrs
  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.
