Implications of the large OVI columns around low-redshift $L_*$ galaxies
  Observations reveal massive amounts of OVI around star-forming $L_*$
galaxies, with covering fractions of near unity extending to the host halo's
virial radius. This OVI absorption is typically kinematically centered upon
photoionized gas, with line widths that are suprathermal and kinematically
offset from the galaxy. We discuss various scenarios and whether they could
result in the observed phenomenology (cooling gas flows, boundary layers,
shocks, virialized gas, photoionized clouds in thermal equilibrium). If
predominantly collisionally ionized, as we argue is most probable, the OVI
observations require that the circumgalactic medium (CGM) of $L_*$ galaxies
holds nearly all the associated baryons within a virial radius ($\sim
10^{11}M_\odot$) and hosts massive flows of cooling gas with
$\approx30[nT/30{\rm~cm^{-3}K}]~M_\odot~$yr$^{-1}$, which must be largely
prevented from accreting onto the host galaxy. Cooling and feedback energetics
considerations require $10 <\langle nT\rangle<100{\rm~cm^{-3}K}$ for the warm
and hot halo gases. We argue that virialized gas, boundary layers, hot winds,
and shocks are unlikely to directly account for the bulk of the OVI.
Furthermore, we show that there is a robust constraint on the number density of
many of the photoionized $\sim10^4$K absorption systems that yields upper
bounds in the range $n<(0.1-3)\times10^{-3}(Z/0.3)$cm$^{-3}$, where $Z$ is the
metallicity, suggestive that the dominant pressure in some photoionized clouds
is nonthermal. This constraint, which requires minimal ionization modeling, is
in accord with the low densities inferred from more complex photoionization
modeling. The large amount of cooling gas that is inferred could re-form these
clouds in a fraction of the halo dynamical time, as some arguments require, and
it requires much of the feedback energy available from supernovae and stellar
winds to be dissipated in the CGM.
