The COS-Halos Survey: Physical Conditions and Baryonic Mass in the
  Low-Redshift Circumgalactic Medium
  We analyze the physical conditions of the cool, photoionized (T $\sim 10^4$
K) circumgalactic medium (CGM) using the COS-Halos suite of gas column density
measurements for 44 gaseous halos within 160 kpc of $L \sim L^*$ galaxies at $z
\sim 0.2$. These data are well described by simple photoionization models, with
the gas highly ionized (n$_{\rm HII}$/n$_{\rm H} \gtrsim 99\%$) by the
extragalactic ultraviolet background (EUVB). Scaling by estimates for the
virial radius, R$_{\rm vir}$, we show that the ionization state (tracked by the
dimensionless ionization parameter, U) increases with distance from the host
galaxy. The ionization parameters imply a decreasing volume density profile
n$_{\rm H}$ = (10$^{-4.2 \pm 0.25}$)(R/R$_{\rm vir})^{-0.8\pm0.3}$. Our derived
gas volume densities are several orders of magnitude lower than predictions
from standard two-phase models with a cool medium in pressure equilibrium with
a hot, coronal medium expected in virialized halos at this mass scale. Applying
the ionization corrections to the HI column densities, we estimate a lower
limit to the cool gas mass M$_{\rm CGM}^{\rm cool} > 6.5 \times 10^{10}$
M$_{\odot}$ for the volume within R $<$ R$_{\rm vir}$. Allowing for an
additional warm-hot, OVI-traced phase, the CGM accounts for at least half of
the baryons purported to be missing from dark matter halos at the 10$^{12}$
M$_{\odot}$ scale.
