Abstract
We construct an analytic phenomenological model for extended warm/hot gaseous
coronae of L* galaxies. We aim to reproduce the column densities of highly
ionized oxygen ions observed in the ultraviolet and X-ray, as evidence for
warm/hot gas in a wide range of temperatures and ionization states. We consider
OVI data from the COS-Halos sample of galaxies in combination with the nearby
OVII and OVIII absorption that we interpret as arising in an extended corona
around the Milky Way. We fit these data sets with a single representative
model. The gas in our model is multiphased, with hot and warm components. Each
component has a (turbulent) log-normal distribution of temperatures and
densities. The hot gas is traced by the OVII and OVIII and is in hydrostatic
equilibrium in a Milky Way gravitational potential. The median temperature of
the hot gas is $1.8*10^6$ K and the resulting mean hydrogen density is $\sim
5*10^-5~cm^-3$, consistent with ram-pressure stripping observed in Milky
Way satellites. The warm component is traced by the OVI as seen in absorption
around the external galaxies. The corona in our model is a large structure,
extending slightly beyond the virial radius. The total warm/hot gas mass is
high and is $1.35*10^11~M_ødot$. The gas metallicity we require to
reproduce the oxygen ion column densities is 0.5 solar. Lower values lead to
higher gas masses, and too many baryons, given the virial mass of the Milky
Way. The warm OVI component has a short cooling time ($<10^8$ years), as hinted
by observations. The hot component, however, is $90\%$ of the total gas
mass and is long-lived, with $t_cool 2*10^10$ years. Our model
suggests that hot coronae of galaxies can contain significant amounts of gas,
enabling galaxies to continue forming stars steadily for long periods of time
and also accounting for "missing baryons" in galaxies in the local universe.
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