Low-mass stars dominate the hot (0.7 keV) Galactic X-ray emission

¹ Osservatorio Astronomico di Brera, (INAF), Via E. Bianchi 46, Merate, 23807, Italy
² Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse, Garching, 85748, Germany
³ Como Lake Center for Astrophysics (CLAP), DiSAT, Università degli Studi dell’Insubria, via Valleggio 11, 22100 Como, Italy
⁴ Institut für Astronomie & Astrophysik, Eberhard Karls Universität Tübingen, Sand 1, Tübingen, 72076, Germany
⁵ Dr. Karl Remeis Observatory, Erlangen Centre for Astroparticle Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg, Sternwartstr. 7, Bamberg, 96049, Germany
⁶ Argelander Institute for Astronomy, University of Bonn, Auf dem Hügel 71, Bonn, 53121, Germany
⁷ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, Potsdam, 14482, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 20 August 2025
Accepted: 17 January 2026

Abstract

Context. The circumgalactic medium of the Milky Way is composed of a tenuous atmosphere filled with multi-phase plasma, including a warm-hot virialised component. Recent studies suggest a much hotter (∼0.7 keV) super-virial component detected in both absorption and emission.

Aims. We want to shed light on the nature of this putative super-virial component.

Methods. We analysed the X-ray background as observed by SRG/eROSITA over the entire western Galactic hemisphere.

Results. We show that low-mass stars provide a large fraction of the 0.7 keV emission. Indeed, a tight correlation is found between the surface brightness of the 0.7 keV emission and the mass distribution of the Milky Way across a large portion of the western Galactic hemisphere. The correlation coefficient implies an X-ray luminosity per unit of stellar mass comparable to that of the average low-mass stars within 10 pc of the Sun, suggesting that unresolved M dwarfs and F, G, and K type stars dominate the 0.7 keV emission. This emission is asymmetric with respect to the Galactic plane, influenced by the asymmetric distribution of nearby star-forming regions, and broadly consistent with the known offset of the Sun above the Galactic midplane. The remaining signal might be produced by the cumulative emission of stars of different types or ages, in addition to other sources (e.g. hot interstellar medium, Galactic corona, etc.). Assuming that the putative residual hot super-virial atmosphere is homogeneous and has a spherical beta profile with slope β = 0.4, we constrain its density at 10 kpc to be n_e < 4 × 10⁻⁴ cm⁻³. Our findings may help refine models of the circumgalactic medium around external galaxies, advancing our understanding of hot baryon flows and galaxy evolution.