X-Ray Absorption from the Milky Way Halo and the Local Group

Million degree gas is present at near-zero redshift and is due to either a gaseous Galactic halo or a more diffuse but very massive Local Group medium. We can discriminate between these models because the column densities should depend on location in the sky, either relative to the Galaxy bulge or to the M31-Milky Way axis. To search for these signatures, we measured the O VII Kα absorption-line strength toward 25 bright active galactic nuclei (AGNs), plus LMC X-3, using XMM-Newton RGS archival data. The data are in conflict with a purely Local Group model, but support the Galactic halo model. The strongest correlation is between the O VII equivalent widths and the ROSAT background emission measurement in the R45 band (0.4-1 keV), for which O VII emission makes the largest single contribution. This suggests that much of the O VII emission and absorption are cospatial, from which the radius of a uniform halo appears to lie the range 15-110 kpc. The present data do not constrain the type of halo gas model, and an equally good fit is obtained in a model where the gas density decreases as a power law, such as r^-3/2. For a uniform halo with a radius of 20 kpc, the electron density would be 9×10^-4 cm^-3, and the gas mass is 4×10⁸ M_solar. The redshift of the four highest signal-to-noise ratio (S/N) O VII measurements is consistent with a Milky Way origin rather than a Local Group origin.