Zooming in on accretion - I. The structure of halo gas
  We study the properties of gas in and around 10^12 solar mass halos at z=2
using a suite of high-resolution cosmological hydrodynamic 'zoom' simulations.
We quantify the thermal and dynamical structure of these gaseous reservoirs in
terms of their mean radial distributions and angular variability along
different sightlines. With each halo simulated at three levels of increasing
resolution, the highest reaching a baryon mass resolution of ~10,000 solar
masses, we study the interaction of filamentary inflow and the quasi-static hot
halo atmosphere. We highlight the discrepancy between the spatial resolution
available in the halo gas as opposed to within the galaxy itself, and find that
stream morphologies become increasingly complex at higher resolution, with
large coherent flows revealing density and temperature structure at
progressively smaller scales. Moreover, multiple gas components co-exist at the
same radius within the halo, making radially averaged analyses misleading. This
is particularly true where the hot, quasi-static, high entropy halo atmosphere
interacts with cold, rapidly inflowing, low entropy accretion. We investigate
the process of gas virialization and identify different regimes for the heating
of gas as it accretes from the intergalactic medium. Haloes at this mass have a
well-defined virial shock, associated with a sharp jump in temperature and
entropy at ~1.25 r_vir. The presence, radius, and radial width of this boundary
feature, however, vary not only from halo to halo, but also as a function of
angular direction, covering roughly ~85% of the 4pi sphere. Our findings are
relevant for the proper interpretation of observations pertaining to the
circumgalactic medium, including evidence for large amounts of cold gas
surrounding massive haloes at intermediate redshifts.
