THE STRUCTURE OF THE MILKY WAY'S HOT GAS HALO
The Milky Way's million degree gaseous halo contains a considerable amount of mass that, depending on its structural properties, can be a significant mass component. In order to analyze the structure of the Galactic halo, we use XMM-Newton Reflection Grating Spectrometer archival data and measure O vii Kα absorption-line strengths toward 26 active galactic nuclei, LMC X-3, and two Galactic sources (4U 1820-30 and X1735-444). We assume a β-model as the underlying gas density profile and find best-fit parameters of n○ = 0.46 cm−3, rc = 0.35 kpc, and β = 0.71. These parameters result in halo masses ranging between M(18 kpc) = 7.5 and M(200 kpc) = 3.8 assuming a gas metallicity of Z = 0.3 Z☉, which are consistent with current theoretical and observational work. The maximum baryon fraction from our halo model of fb = 0.07 is significantly smaller than the universal value of fb = 0.171, implying the mass contained in the Galactic halo accounts for 10%–50% of the missing baryons in the Milky Way. We also discuss our model in the context of several Milky Way observables, including ram pressure stripping in dwarf spheroidal galaxies, the observed X-ray emission measure in the 0.5–2 keV band, the Milky Way's star formation rate, spatial and thermal properties of cooler gas (∼105 K), and the observed Fermi bubbles toward the Galactic center. Although the metallicity of the halo gas is a large uncertainty in our analysis, we place a lower limit on the halo gas between the Sun and the Large Magellanic Cloud (LMC). We find that Z ≳ 0.2 Z☉ based on the pulsar dispersion measure toward the LMC.