Abundance and temperature of the outer hot circumgalactic medium The SRG/eROSITA view of the soft X-ray background in the eFEDS field

Context. Despite their vital importance to understanding galaxy evolution and our own Galactic ecosystem, our knowledge of the physical properties of the hot X-ray emitting phase of the Milky Way is still inadequate. However, sensitive SRG/eROSITA large area surveys are now providing us with the long-sought data needed to mend this state of affairs. Aims. Our aim is to constrain the properties of the Milky Way hot halo emission toward intermediate Galactic latitudes close to the Galactic anti-center. Methods. We analyzed the spectral properties of the integrated soft X-ray emission observed by eROSITA in the relatively deep eFEDS field. Results. We observe a flux of 12.6 and 5.1 x 10(-12) erg cm(-2) s(-1) deg(-2) in the total (0.3-2) and soft (0.3-0.6 keV) band. We measure the temperature and metal (oxygen) abundance of the hot circumgalactic medium (CGM) to be within kT(CGM) = 0.153-0.178 keV and Z(CGM) = 0.052-0.072 Z(circle dot), depending on the contribution of solar wind charge exchange (SWCX). Slightly higher CGM abundances Z(CGM) = 0.05-0.10 Z(circle dot) are possible, considering the uncertain extrapolation of the extragalactic cosmic X-ray background (CXB) emission below similar to 1 keV. To recover CGM abundances as high as Z(CGM) = 0.3 Z(circle dot), the presence of an additional component must be postulated, likely associated with the warm-hot intergalactic medium, providing similar to 15-20% of the flux in the soft X-ray band. We observe line widths of the CGM plasma smaller than Delta v <= 500 km s(-1). The emission in the soft band is dominated (similar to 47%) by the circumgalactic medium (CGM), whose contribution reduces to similar to 30% if heliospheric SWCX contributes at the level of similar to 15% also during solar minimum. The remaining flux is provided by the CXB (similar to 33%) and the local hot bubble (similar to 18%). Moreover, the eROSITA data require the presence of an additional component associated with the elusive Galactic corona plus a possible contribution from unresolved M dwarf stars. This component has a temperature of kT similar to 0.4-0.7 keV, a considerable (similar to kiloparsec) scale height, and might be out of thermal equilibrium. It contributes similar to 9% to the total emission in the 0.6-2 keV band, and is therefore a likely candidate to produce part of the unresolved CXB flux observed in X-ray ultra-deep fields. We also observe a significant contribution to the soft X-ray flux due to SWCX, during periods characterized by stronger solar wind activity, and causing the largest uncertainty on the determination of the CGM temperature. Conclusions. We constrain temperature, emission measure, abundances, thermal state, and spectral shape of the outer hot CGM of the Milky Way.

Automated summary

By analyzing the data from the SRG/eROSITA mission, we have successfully constrained the temperature, elemental abundance, and spectral shape of the hot halo emission in the Milky Way. Our study reveals that the circumgalactic medium (CGM) dominates the soft X-ray flux, contributing approximately 47% of the total emission. Additionally, we observe the presence of an extra component likely associated with the Galactic corona and unresolved M dwarf stars.