The Baryonic Assembly of Dark Matter Halos
  We use a suite of cosmological hydrodynamic simulations to quantify the
accretion rates of baryons into dark matter halos and the resulting baryon mass
fractions, as a function of halo mass, redshift, and baryon type (including
cold and hot gas). We find that the net baryonic accretion rates through the
virial radius are sensitive to galactic outflows and explore a range of outflow
parameters to illustrate the effects. We show that the cold gas accretion rate
is in general not a simple universal factor of the dark matter accretion rate,
and that galactic winds can cause star formation rates to deviate significantly
from the external gas accretion rates, both via gas ejection and re-accretion.
Furthermore, galactic winds can inject enough energy and momentum in the
surrounding medium to slow down accretion altogether, especially in low-mass
halos and at low redshift. By resolving the accretion rates versus radius from
the halo centers, we show how cold streams penetrate the hot atmospheres of
massive halos at z>2, but gradually disappear at lower redshift. The total
baryon mass fraction is also strongly suppressed by outflows in low-mass halos,
but is nearly universal in the absence of feedback in halos above the UV
background suppression scale. The transition halo mass, at which the gas mass
in halos is equal for the cold and hot components, is roughly constant at
~10^11.5 Msun and does not depend sensitively on the wind prescription. We
provide simple fitting formulae for the cold gas accretion rate into halos in
the no-wind case. Finally, we show that cold accretion is broadly consistent
with driving the bulk of the highly star-forming galaxies observed at z~2, but
that the more intense star formers likely sample the high end of the accretion
rate distribution, and may be additionally fueled by a combination of gas
recycling, gas re-accretion, hot mode cooling, and mergers.
