Cold accretion flows and the nature of high column density HI absorption
  at redshift 3
  Simulations predict that galaxies grow primarily through the accretion of gas
that has not gone through an accretion shock near the virial radius and that
this cold gas flows towards the central galaxy along dense filaments and
streams. There is, however, little observational evidence for the existence of
these cold flows. We use a large, cosmological, hydrodynamical simulation that
has been post-processed with radiative transfer to study the contribution of
cold flows to the observed z=3 column density distribution of neutral hydrogen,
which our simulation reproduces. We find that nearly all of the HI absorption
arises in gas that has remained colder than 10^5.5 K, at least while it was
extragalactic. In addition, the majority of the HI is rapidly falling towards a
nearby galaxy, with non-negligible contributions from outflowing and static
gas. Above a column density of N_HI = 10^17 cm^-2, most of the absorbers reside
inside haloes, but the interstellar medium only dominates for N_HI > 10^21
cm^-2. Haloes with total mass below 10^10 Msun dominate the absorption for
10^17<N_HI < 10^21 cm^-2, but the average halo mass increases sharply for
higher column densities. Although very little of the HI in absorbers with N_HI
<~ 10^20 cm^-2 resides inside galaxies, systems with N_HI > 10^17 cm^-2 are
closely related to star formation: most of their HI either will become part of
the interstellar medium before z=2 or has been ejected from a galaxy at z>3.
Cold accretion flows are critical for the success of our simulation in
reproducing the observed rate of incidence of damped Lyman-alpha and
particularly that of Lyman limit systems. We therefore conclude that cold
accretion flows exist and have already been detected in the form of high column
density HI absorbers.
