Damped Ly{\alpha} Absorption Systems in Semi-Analytic Models with
  Multiphase Gas
  We investigate the properties of damped Ly{\alpha} absorption systems (DLAs)
in semi-analytic models of galaxy formation, including partitioning of cold gas
in galactic discs into atomic, molecular, and ionized phases with a molecular
gas-based star formation recipe. We investigate two approaches for partitioning
gas into these constituents: a pressure-based and a metallicity-based recipe.
We identify DLAs by passing lines of sight through our simulations to compute
HI column densities. We find that models with "standard" gas radial profiles -
where the average specific angular momentum of the gas disc is equal to that of
the host dark matter halo - fail to reproduce the observed column density
distribution of DLAs. These models also fail to reproduce the distribution of
velocity widths {\Delta}v, overproducing low {\Delta}v relative to high
{\Delta}v systems. Models with "extended" radial gas profiles - corresponding
to gas discs with higher specific angular momentum - are able to reproduce
quite well the column density distribution of absorbers over the column density
range 19 < log NHI < 22.5 in the redshift range 2 < z < 3.5. The model with
pressure-based gas partitioning also reproduces the observed line density of
DLAs, HI gas density, and {\Delta}v distribution at z < 3 remarkably well.
However all of the models investigated here underproduce DLAs and the HI gas
density at z > 3. If this is the case, the flatness in the number of DLAs and
HI gas density over the redshift interval 0 < z < 5 may be due to a cosmic
coincidence where the majority of DLAs at z > 3 arise from intergalactic gas in
filaments while those at z < 3 arise predominantly in galactic discs. We
further investigate the dependence of DLA metallicity on redshift and
{\Delta}v, and find reasonably good agreement with the observations,
particularly when including the effects of metallicity gradients (abbrv.).
