Probing the Anisotropy of the Milky Way Gaseous Halo-II: sightline toward Mrk509

TL;DR

This study investigates the anisotropic hot gaseous halo of the Milky Way along the sightline to Mrk509, combining X-ray emission and absorption data to characterize its density, temperature, and mass, revealing a substantial circumgalactic medium.

Contribution

It provides new measurements of the Milky Way's CGM emission and absorption along a specific sightline, modeling its density profile and estimating its mass, highlighting the anisotropic nature of the halo.

Findings

Emission measure is five times higher than average.

Gas temperature is approximately 2 million Kelvin.

Minimum hot CGM mass is about 3.2 x 10^{10} solar masses.

Abstract

Hot, million degree gas appears to pervade the Milky way halo, containing a large fraction of the Galactic missing baryons. This circumgalactic medium (CGM) is probed effectively in X-rays, both in absorption and in emission. The CGM also appears to be anisotropic, so we have started a program to determine CGM properties along several sightlines by combining absorption and emission measurements. Here we present the emission measure close to the \mrk509 sightline using new \suzaku and \xmm observations. We also present new analysis and modeling of \chandra HETG spectra to constrain the absorption parameters. The emission measure in this sightline is high, EM$=0.0165\pm0.0008\pm0.0006~$cm$^{-6}~$pc, five times larger than the average. The observed \ovii column density N(\ovii)$= 2.35\pm0.4 \times 10^{16}$cm$^{-2}$, however, is close to the average. We find that the temperature of the emitting and absorbing gas is the same: $\log T (\rm K) = 6.33\pm0.01$ and $\log T (\rm K)=6.33\pm0.16$ respectively. We fit the observed column density and emission measure with a $\beta-$model density profile. The the central density is constrained to be between $n_0=2.8$--$6.0\times 10^{-4}$ cm$^{-3}$ and the core radius of the density profile has a lower limit of 40 kpc. This shows that the hot gas is mostly in the CGM of the galaxy, not in the Galactic disk. Our derived density profile is close to the \citet{Maller2004} profile for adiabatic gas in hydrostatic equilibrium with an NFW dark matter potential well. Assuming this density profile, the minimum mass of the hot CGM is $3.2 \times 10^{10}~$M$_{\odot}$.