The Physical Nature of the Most Metal-Poor Damped Lyman Alpha Systems
  Utilizing the high-resolution, large-scale LAOZI cosmological simulations we
investigate the nature of the metal-poor (${\rm [Z/H]<-2}$) damped Lyman alpha
systems (mpDLA) at $z=3$. The following physical picture of mpDLAs emerges. The
majority of mpDLAs inhabit regions $\ge 20$~kpc from the host galaxy center on
infalling cold gas streams originating from the intergalactic medium, with
infall velocity of $\sim 100$ km/s and temperature of $\sim 10^{4}$ K. For each
host galaxy, on average, about $1\%$ of the area within a radius $150$~kpc is
covered by mpDLAs. The mpDLAs are relatively diffuse ($n_{\rm{gas}} \sim
10^{-2}$ cm$^{-3}$), Jeans quasi-stable, and have very low star formation rate
($\dot{\Sigma} \le 10^{-4} M_{\odot} \rm{\ yr}^{-1} \rm{\ kpc}^{-2}$). As
mpDLAs migrate inward to the galaxy center, they mix with high metallicity gas
and stellar outflows in the process, removing themselves from the metal-poor
category and rendering the central ($\le 5$ kpc) regions of galaxies devoid of
mpDLAs. Thus, the central regions of the host galaxies are populated by mostly
metal-rich DLAs instead of mpDLAs. All observables of the simulated mpDLAs are
in excellent agreement with observations, except the gas density, which is
about a factor of ten lower than the value inferred observationally. However,
the observationally inferred value is based on simplified assumptions that are
not borne out in the simulations.
