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 K \alpha absorption-line strengths toward 26 active galactic nuclei , 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 _ { \circ } = 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 } \times 10 ^ { 8 } M _ { \sun } and M ( 200 kpc ) = 3.8 ^ { +6.0 } _ { -0.5 } \times 10 ^ { 10 } M _ { \sun } assuming a gas metallicity of Z = 0.3 Z _ { \odot } , 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 ( \sim 10 ^ { 5 } 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 \gtrsim 0.2 Z _ { \odot } based on the pulsar dispersion measure toward the LMC .