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 M31YMilky Way axis. To search for these signatures, we measured the O vii K absorption-line strength toward 25 bright active galactic nuclei (AGNs), plus LMCX-3, using XMM-NewtonRGS archival data. The data are in conflict with a purely Local Groupmodel, but sup-port theGalactic halomodel. The strongest correlation is between theO vii equivalent widths and theROSAT background emission measurement in the R45 band (0.4Y1 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 15Y110 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 r3/2. For a uniform halo with a radius of 20 kpc, the electron density would be 9; 104 cm3, and the gasmass is 4; 108 M. The redshift of the four highest signal-to-noise ratio (S/N)O viimeasurements is consistent with aMilkyWay origin rather than a Local Group origin.