The Milky Way hosts a hot ( \approx 2 \times 10 ^ { 6 } K ) , diffuse , gaseous halo based on detections of z = 0 O vii and O viii absorption lines in quasar spectra and emission lines in blank-sky spectra .
Here we improve constraints on the structure of the hot gas halo by fitting a radial model to a much larger sample of O vii and O viii emission line measurements from XMM-Newton /EPIC-MOS spectra compared to previous studies ( \approx 650 sightlines ) .
We assume a modified \beta -model for the halo density distribution and a constant-density Local Bubble from which we calculate emission to compare with the observations .
We find an acceptable fit to the O viii emission line observations with \chi ^ { 2 } _ { red } ( dof ) = 1.08 ( 644 ) for best-fit parameters of n _ { o } r _ { c } ^ { 3 \beta } = 1.35 \pm 0.24 cm ^ { -3 } kpc ^ { 3 \beta } and \beta = 0.50 \pm 0.03 for the hot gas halo and negligible Local Bubble contribution .
The O vii observations yield an unacceptable \chi ^ { 2 } _ { red } ( dof ) = 4.69 ( 645 ) for similar best-fit parameters , which is likely due to temperature or density variations in the Local Bubble .
The O viii fitting results imply hot gas masses of M ( \textless 50 kpc ) = 3.8 _ { -0.3 } ^ { +0.3 } \times 10 ^ { 9 } M _ { \odot } and M ( \textless 250 kpc ) = 4.3 _ { -0.8 } ^ { +0.9 } \times 10 ^ { 10 } M _ { \odot } , accounting for \lesssim 50 % of the Milky Way ’ s missing baryons .
We also explore our results in the context of optical depth effects in the halo gas , the halo gas cooling properties , temperature and entropy gradients in the halo gas , and the gas metallicity distribution .
The combination of absorption and emission line analyses implies a sub-solar gas metallicity that decreases with radius , but that also must be \geq 0.3 Z _ { \odot } to be consistent with the pulsar dispersion measure toward the Large Magellanic Cloud .