Denis Erkal, Nickolay Y. Gnedin, Andrey V. Kravtsov
We study the high column density regime of the HI column density distribution
function and argue that there are two distinct features: a turnover at NHI ~
10^21 cm^-2 which is present at both z=0 and z ~ 3, and a lack of systems above
NHI ~ 10^22 cm^-2 at z=0. Using observations of the column density
distribution, we argue that the HI-H2 transition does not cause the turnover at
NHI ~ 10^21 cm^-2, but can plausibly explain the turnover at NHI > 10^22 cm^-2.
We compute the HI column density distribution of individual galaxies in the
THINGS sample and show that the turnover column density depends only weakly on
metallicity. Furthermore, we show that the column density distribution of
galaxies, corrected for inclination, is insensitive to the resolution of the HI
map or to averaging in radial shells. We show that observed HI column density
distribution at high NHI is consistent with radial HI profiles of the THINGS
galaxies averaged over all possible inclinations. Our results indicate that the
similarity of HI column density distributions at z=3 and z=0 is due to the
similarity of the maximum HI surface densities of high-z and low-z disks, set
presumably by universal processes that shape properties of the gaseous disks of
galaxies. Using fully cosmological simulations, we explore other candidate
physical mechanisms that could produce a turnover in the column density
distribution. We show that while turbulence within GMCs cannot affect the DLA
column density distribution, stellar feedback can affect it significantly if
the feedback is sufficiently effective in removing gas from the central 2-3 kpc
of high-redshift galaxies. Finally, we argue that it is meaningful to compare
column densities averaged over ~ kpc scales with those estimated from quasar
spectra which probe sub-pc scales due to the steep power spectrum of HI column
density fluctuations observed in nearby galaxies. (Abridged)
View original:
http://arxiv.org/abs/1201.3653
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