Ambient Column Densities of Highly Ionized Oxygen in
  Precipitation-Limited Circumgalactic Media
  Many of the baryons associated with a galaxy reside in its circumgalactic
medium (CGM), in a diffuse volume-filling phase at roughly the virial
temperature. Much of the oxygen produced over cosmic time by the galaxy's stars
also ends up there. The resulting absorption lines in the spectra of UV and
X-ray background sources are powerful diagnostics of the feedback processes
that prevent more of those baryons from forming stars. This paper presents
predictions for CGM absorption lines (O VI, O VII, O VIII, Ne VIII, N V) that
are based on precipitation-regulated feedback models, which posit that the
radiative cooling time of the ambient medium cannot drop much below 10 times
the freefall time without triggering a strong feedback event. The resulting
predictions align with many different observational constraints on the Milky
Way's ambient CGM and explain why N_OVI ~ 10^14 cm^-2 over large ranges in halo
mass and projected radius. Within the precipitation framework, the strongest
O~VI absorption lines result from vertical mixing of the CGM that raises
low-entropy ambient gas to greater altitudes, because adiabatic cooling of the
uplifted gas then lowers its temperature and raises the fractional abundance of
O^5+. Condensation stimulated by uplift may also produce associated
low-ionization components. The observed velocity structure of the O VI
absorption suggests that galactic outflows do not expel circumgalactic gas at
the halo's escape velocity but rather drive circulation that dissipates much of
the galaxy's supernova energy within the ambient medium, causing some of it to
expand beyond the virial radius.
