The Statistical Properties of Neutral Gas at z<1.65 from UV Measurements
  of Damped Lyman Alpha Systems
  We derive the statistical properties of neutral gas at redshifts 0.11<z<1.65
from UV measurements of quasar Lyman alpha absorption lines corresponding to
369 MgII systems with $W^{\lambda2796}_{0} \ge 0.3$ \AA. In addition to the 41
damped Lyman alpha (DLA) systems presented in Rao et al. (2006), the current
DLA sample includes 29 newly discovered DLAs. Of these, 26 were found in our
HST ACS prism survey for DLAs (Turnshek et al. 2015) and three were found in a
GALEX archival search. In addition, an HST COS Cycle 19 survey yielded no DLAs
that could be used for this study. Formally, this DLA sample includes 70
systems with $N_{\rm HI}\ge 2\times 10^{20}$ atoms cm$^{-2}$. We find that the
incidence of DLAs, or the product of their gas cross section and their comoving
number density, can be described by $n_{\rm DLA}(z) = (0.027 \pm 0.007)
(1+z)^{(1.682 \pm 0.200)}$ over the redshift range 0<z<5. The cosmic mass
density of neutral gas can be described by $\Omega_{\rm DLA}(z) = (4.77 \pm
1.60)\times10^{-4} (1 + z)^{(0.64\pm 0.27)}$. The low-redshift column density
distribution function is well-fitted by a power law of the form $f(N) \sim
N^\beta$ with $\beta = -1.46 \pm 0.20$. It is consistent with the high-redshift
as well as z=0 estimates at the high column density end but, lies between them
at the low column density end. We discuss possible $N_{\rm HI}$ and metallicity
bias in MgII-selected DLA samples and show that such biases do not exist in the
current data at z<1.65. Thus, at least at z<1.65, DLAs found through MgII
selection statistically represent the true population of DLAs. However, we
caution that studies of DLA metallicities should take into the account the
relative incidence of DLAs with respect to $W^{\lambda2796}_{0}$ (or gas
velocity spread) in order to correctly measure the mean neutral-gas cosmic
metallicity of the universe.
