The Structure of the Milky Way's Hot Gas Halo

Abstract

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 _circle = 0.46^{+0.74}_{-0.35} cm^-3, r_c = 0.35^{+0.29}_{-0.27} kpc, and β = 0.71^{+0.13}_{-0.14}. These parameters result in halo masses ranging between M(18 kpc) = 7.5^{+22.0}_{-4.6} \times 10^8\ M_\odot and M(200 kpc) = 3.8^{+6.0}_{-0.5} \times 10^{10}\ M_\odot 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 f_b = 0.07^{+0.03}_{-0.01} is significantly smaller than the universal value of f_b = 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.