Misc,
The Structure of the Milky Way's Hot Gas Halo
(2013)cite arxiv:1305.2430Comment: 33 pages, 10 figures, accepted to ApJ.
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 RGS archival data and measure OVII K alpha absorption-line strengths
towards 26 active galactic nuclei (AGN), LMC X-3, and two Galactic sources (4U
1820-30 and X1735-444). We assume a beta-model as the underlying gas density
profile and find best-fit parameters of n_o = 0.46^+0.74_-0.35 cm^-3, r_c =
0.35^+0.29_-0.27 kpc, and beta = 0.71^+0.13_-0.14. These parameters
result in halo masses ranging between M(18 kpc) = 7.5^+22.0_-4.6 x 10^8
M_sun and M(200 kpc) = 3.8^+6.0_-0.5 x 10^10 M_sun assuming a gas
metallicity of Z = 0.3 Z_sun, 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 (~10^5 K) and the observed Fermi bubbles towards 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 LMC.
We find that Z >~ 0.2 Z_sun based on the pulsar dispersion measure towards the
LMC.
