Baryon Cycling in the Low-Redshift Circumgalactic Medium: A Comparison
  of Simulations to the COS-Halos Survey
  We analyze the low-redshift (z~0.2) circumgalactic medium by comparing
absorption-line data from the COS-Halos Survey to absorption around a matched
galaxy sample from two cosmological hydrodynamic simulations. The models
include different prescriptions for galactic outflows, namely hybrid
energy/momentum driven wind (ezw), and constant winds (cw). We extract for
comparison direct observables including equivalent widths, covering factors,
ion ratios, and kinematics. Both wind models are generally in good agreement
with these observations for HI and certain low ionization metal lines, but show
poorer agreement with higher ionization metal lines including SiIII and OVI
that are well-observed by COS-Halos. These discrepancies suggest that both wind
models predict too much cool, metal-enriched gas and not enough hot gas, and/or
that the metals are not sufficiently well-mixed. This may reflect our model
assumption of ejecting outflows as cool and unmixing gas. Our ezw simulation
includes a heuristic prescription to quench massive galaxies by super-heating
its ISM gas, which we show yields sufficient low ionisation absorption to be
broadly consistent with observations, but also substantial OVI absorption that
is inconsistent with data, suggesting that gas around quenched galaxies in the
real Universe does not cool. At impact parameters of <50 kpc, recycling winds
dominate the absorption of low ions and even HI, while OVI almost always arises
from metals ejected longer than 1 Gyr ago. The similarity between the wind
models is surprising, since we show that they differ substantially in their
predicted amount and phase distribution of halo gas. We show that this
similarity owes mainly to our comparison here at fixed stellar mass rather than
at fixed halo mass in our previous works, which suggests that CGM properties
are more closely tied to the stellar mass of galaxies rather than halo mass.
