On the Hot Gas Content of the Milky Way Halo

Abstract: The Milky Way appears to be missing baryons, as the observed mass in stars and gas is well below the cosmic mean. One possibility is that a substantial fraction of the Galaxy's baryons are embedded within an extended, million-degree hot halo, an idea supported indirectly by observations of warm gas clouds in the halo and gas-free dwarf spheroidal satellites. X-ray observations have established that hot gas does exist in our Galaxy beyond the local hot bubble; however, it may be distributed in a hot disk configuration. Moreover, recent investigations into the X-ray constraints have suggested that any Galactic corona must be insignificant. Here we re-examine the observational data, particularly in the X-ray and radio bands, in order to determine whether it is possible for a substantial fraction of the Galaxy's baryons to exist in ~ 10^6 K gas. In agreement with past studies, we find that a baryonically closed halo is clearly ruled out if one assumes that the hot corona is distributed with a cuspy NFW profile. However, if the hot corona of the galaxy is in an extended, low-density distribution with a large central core, as expected for an adiabatic gas in hydrostatic equilibrium, then it may contain up to 10^11 M_sun of material, possibly accounting for all of the missing Galactic baryons. We briefly discuss some potential avenues for discriminating between a massive, extended hot halo and a local hot disk.