Spatial Distribution of the Milky Way Hot Gaseous Halo Constrained by Suzaku X-Ray Observations

Abstract

The formation mechanism of the hot gaseous halo associated with the Milky Way Galaxy is still under debate. We report new observational constraints on the gaseous halo using 107 lines-of-sight of the Suzaku X-ray observations at 75 • < l < 285 • and |b| > 15 • with a total exposure of 6.4 Ms. The gaseous halo spectra are represented by a single-temperature plasma model in collisional ionization equilibrium. The median temperature of the observed fields is 0.26 keV (3.0 × 10 6 K) with a typical fluctuation of ∼ 30%. The emission measure varies by an order of magnitude and marginally correlates with the Galactic latitude. Despite the large scatter of the data, the emission measure distribution is roughly reproduced by a disk-like density distribution with a scale length of ∼ 7 kpc, a scale height of ∼ 2 kpc, and a total mass of ∼ 5 × 10 7 M . In addition, we found that a spherical hot gas with the β-model profile hardly contributes to the observed X-rays but that its total mass might reach 10 9 M . Combined with indirect evidence of an extended gaseous halo from other observations, the hot gaseous halo likely consists of a dense disk-like component and a rarefied spherical component; the X-ray emissions primarily come from the former but the mass is dominated by the latter. The disk-like component likely originates from stellar feedback in the Galactic disk due to the low scale height and the large scatter of the emission measures. The median [O/Fe] of ∼ 0.25 shows the contribution of the core-collapse supernovae and supports the stellar feedback origin.

Multiple Temperature Components of the Hot Circumgalactic Medium of the Milky Way

“…The temperature of the warm-hot component is in excellent agreement with earlier emission and absorption measurements along many other directions Gupta et al 2017), and is consistent with the most likely temperature (Henley et al 2010;Gupta et al 2012;Henley & Shelton 2013Nakashima et al 2018) of the warm-hot halo. The emission measure of the warm-hot component is a ∼ factor of 2 higher than the average (Henley et al 2010), but is similar to the emission measure along the Mrk 509 sightline found by Gupta et al (2017), and well within the large range of EM spanning ∼ factor of 50 , their figure 7, for a comprehensive picture).…”

“…Taking into account the different energy coverage of detectors, differences in methodologies and model descriptions, and different spectral aspects of emitting and absorbing components, we infer that the X-ray traced CGM has three (T1, T2, T 3 ) or four (T1, T2, T3, T4) phases/components. different from the 10 6.3−6.4 K phase, which is obtained by most of the emission and absorption studies Gupta et al 2012;Henley & Shelton 2013;Gatuzz & Churazov 2018;Nakashima et al 2018). The hotter components have not been observed earlier in general, likely due to shallower data and/or poor spectral resolution.…”

“…Ours is the first detection of the two-temperature (hot and warm-hot) halo gas; these were not detected previously in emission or absorption along other sightlines. Nakashima et al (2018) detected a similarly hot halo component in emission along some anti-center directions, but their halo model had only one temperature component. Henley & Shelton (2013) found an excess emission around 1 keV toward the sightline of NGC 1365 (their sightline 83, figure 2) and on a small number of other sightlines.…”

MAXI/SSC all-sky maps from 0.7 keV to 4 keV

“…Based on their results, we estimate the 0.7-2 keV CXB surface brightness as 6.3 × 10 −8 erg cm −2 s −1 sr −1 , and the brightness variations as 3% and 0.3% for the PSF of the SSC and a circle of 15 • radius, respectively. The GH component was studied by Nakashima et al (2018) in detail using the Suzaku XIS data, mainly on the Galactic anti-center hemisphere. Its spectrum is expressed by a thin thermal plasma model with a temperature of ∼ 0.26 keV.…”

“…We employed the spectral model for the CXB as already described. The GH emission was represented by an APEC plasma model in Xspec, with the plasma temperature of kT = 0.26 keV, the metallicity of Z = 1Z ⊙ , and an emission measure of 0.31 × 10 −2 cm −6 pc (median of Nakashima et al 2018). In figure 3, these components are shown by the green and cyan lines, respectively.…”