Constraining the Milky Way's Hot Gas Halo With O Vii and O Viii Emission Lines

Abstract

The Milky Way hosts a hot (≈ 2 × 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 (≈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 O VIII emission line observations with χ 2 red (dof) = 1.08 (644) for best-fit parameters of n o r 3β c = 1.35 ± 0.24 cm −3 kpc 3β and β = 0.50 ± 0.03 for the hot gas halo and negligible Local Bubble contribution. The O VII 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 O VIII fitting results imply hot gas masses of M (<50 kpc) = 3.8 +0.3 −0.3 × 10 9 M ⊙ and M (<250 kpc) = 4.3 +0.9 −0.8 × 10 10 M ⊙ , accounting for 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 ≥ 0.3Z ⊙ to be consistent with the pulsar dispersion measure toward the Large Magellanic Cloud.

The Rotation of the Hot Gas Around the Milky Way

“…This would impact the inferred mass and total angular momentum of the halo gas. Constraints from OVII and OVIII and pulsar dispersion measures toward the Large Magellanic Cloud are consistent (Miller & Bregman 2015) with a gradient of ( ) µ -Z r r 0.2 with…”

“…We adopted an extended density profile (Miller & Bregman 2015) in which (Cen 2012). We also account for the possibility of intrinsic scatter (resulting from hydrodynamic flows) about any model by adding a velocity dispersion to the velocities when comparing to the models.…”

“…Based on work over the past several years, we know that these extended halos exist, including around the Milky Way (Anderson & Bregman 2011;Anderson et al 2013;Miller & Bregman 2015). The extent and luminosity of the hot gas implies that it has a similar mass to the stellar disk, and therefore could play an important role in galaxy evolution.…”

“…The λ21.602 Å resonance O VII absorption line (Drake 1988) is the best candidate, since it is sensitive to temperatures of -10 10 5.5 6.3 K (which includes much of the Galactic coronal gas) and it is detected at zero redshift toward a large number of background continuum sources (e.g., Fang et al 2015). The emission lines produced by the same species are useful for determining the structure and temperature of the hot gas (Henley & Shelton 2012;Miller & Bregman 2015), but they are too faint for high-resolution spectroscopy. In this paper we constrain, for the first time, the radial and azimuthal velocity of the hot gas by measuring the Doppler shifts in O VII lines detected toward bright sources outside the disk of the Milky Way.…”

Taking care of business in a flash : constraining the time-scale for low-mass satellite quenching with ELVIS

“…Underlying this hypothesis is the notion that the Milky Way and M31 both harbor extended (∼ 150 kpc) reservoirs of hot baryons with sufficient density to strip small galaxies of their gas. There is evidence that M31 does indeed host an extended circumgalactic medium (Lehner, Howk & Wakker 2015), and X-ray studies are consistent with this possibility around the Milky Way (Fang, Bullock & Boylan-Kolchin 2013;Miller & Bregman 2015;Fang et al 2015). If extended low-density coronae of this kind are common around ∼ L * galaxies, then they may prove important for understanding the baryon cycle and the global census of baryons in the Universe.…”