The Small Covering Factor of Cold Accretion Streams
  Theoretical models of galaxy formation predict that galaxies acquire most of
their baryons via cold mode accretion. Observations of high-redshift galaxies,
while showing ubiquitous outflows, have so far not revealed convincing traces
of the predicted cold streams, which has been interpreted as a challenge for
the current models. Using high-resolution, zoom-in smooth particle
hydrodynamics simulations of Lyman break galaxy (LBG) halos combined with
ionizing radiative transfer, we quantify the covering factor of the cold
streams at z=2-4. We focus specifically on Lyman limit systems (LLSs) and
damped Ly-alpha absorbers (DLAs), which can be probed by absorption
spectroscopy using a background galaxy or quasar sightline, and which are
closely related to low-ionization metal absorbers. We show that the covering
factor of these systems is relatively small and decreases with time. At z=2,
the covering factor of DLAs within the virial radius of the simulated galaxies
is ~3% (~1% within twice this projected distance), and arises principally from
the galaxy itself. The corresponding values for LLSs are ~10% and ~4%. Because
of their small covering factor compared to the order unity covering fraction
expected for galactic winds, the cold streams are naturally dominated by
outflows in stacked spectra. We conclude that the existing observations are
consistent with the predictions of cold mode accretion, and outline promising
kinematic and chemical diagnostics to separate out the signatures of galactic
accretion and feedback.
