Constraining the Milky Way's Hot Gas Halo with OVII and OVII Emission Lines

Abstract

The Milky Way hosts a hot ($\approx 2 \times 10^6$ K), diffuse, gaseous halo based on detections of z = 0 OVII and OVIII 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 OVII and OVIII emission line measurements from XMM-Newton EPIC-MOS spectra compared to previous studies ($\approx$ 650 sightlines). We assume a modified $β$-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 OVIII emission line observations with $χ^{2}_{red}$ (dof) = 1.08 (644) for best-fit parameters of $n_o r_c^{3β} = 1.35 \pm 0.24$ cm$^{-3}$ kpc$^{3β}$ and $β= 0.50 \pm 0.03$ for the hot gas halo and negligible Local Bubble contribution. The OVII observations yield an unacceptable $χ^{2}_{red}$ (dof) = 4.69 (645) for similar best-fit parameters, which is likely due to temperature or density variations in the Local Bubble. The OVIII fitting results imply hot gas masses of $M$(< 50 kpc) = $3.8_{-0.3}^{+0.3} \times 10^{9} M_{\odot}$ and $M$(< 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 LMC.