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

Abstract

Despite their vital importance to understand galaxy evolution and our own Galactic ecosystem, our knowledge of the physical properties of the hot phase of the Milky Way is still inadequate. However, sensitive SRG/eROSITA large area surveys are now providing us with the long sought-after data needed to mend this state of affairs. We present the properties of the soft X-ray emission as observed by eROSITA in the eFEDS field. We measure the temperature and metal abundance of the hot circum-Galactic medium (CGM) to be within $k{T}_{CGM}=0.153-0.178$ keV and $Z_{CGM}=0.052-0.072$ $Z_{\odot }$, depending on the contribution of solar wind charge exchange (SWCX). Slightly larger CGM abundances $Z_{CGM}=0.05-0.10$ $Z_{\odot }$ are possible, considering the uncertain extrapolation of the extragalactic Cosmic X-ray background (CXB) emission below $\sim 1$ keV. To recover CGM abundances as large as $Z_{CGM}=0.3$ $Z_{\odot }$, it must be postulated the presence of an additional component, likely associated with the warm-hot intergalactic medium, providing $\sim 15-20$% of the flux in the soft X-ray band. The emission in the soft band is dominated by the CGM, with contributions from the CXB and the local hot bubble. 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 $kT\sim 0.4-0.7$ keV and it might be out of thermal equilibrium. It contributes $\sim 9$% to the total emission in the 0.6--2 keV band, therefore it is 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 characterised by stronger solar wind activity, and causing the largest uncertainty on the determination of the CGM temperature.

Accepted for publication in Astronomy and Astrophysics

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