On the baryon budget in the X-ray-emitting circumgalactic medium of Milky Way-mass galaxies

¹ Max-Planck-Institut für extraterrestrische Physik (MPE) Gießenbachstraße 1 D-85748 Garching bei München, Germany
² INAF-Osservatorio Astronomico di Brera Via E. Bianchi 46 I-23807 Merate (LC), Italy
³ Como Lake Center for Astrophysics (CLAP), DiSAT, Università degli Studi dell’Insubria via Valleggio 11 22100 Como, Italy
⁴ Department of Astronomy, Tsinghua University Beijing 100084, People’s Republic of China
⁵ Purple Mountain Observatory, Chinese Academy of Sciences 10 Yuanhua Road Nanjing 210023, People’s Republic of China
⁶ Department of Astronomy, University of Michigan Ann Arbor MI 48109, USA
⁷ Department of Astronomy, Xiamen University Xiamen Fujian 361005, China

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

Received: 12 August 2025
Accepted: 22 November 2025

Abstract

Recent observations with SRG/eROSITA reveal the average X-ray surface brightness profile of the X-ray-emitting circumgalactic medium (CGM) around Milky Way (MW)-mass galaxies, offering valuable insights into the baryon budget in these systems. However, the estimation of the baryon mass depends critically on several assumptions regarding the gas density profile, temperature, metallicity, and the underlying halo mass distribution. Here, we assess how these assumptions affect the inferred baryon mass of the X-ray-emitting CGM in MW-mass galaxies, based on the stacked eROSITA signal. We find that variations in temperature profiles and uncertainties in the halo mass introduce the dominant sources of uncertainty, resulting in X-ray-emitting baryon mass estimates that vary by nearly a factor of four (0.8–3.5 × 10¹¹ M_⊙). Assumptions about metallicity contribute an additional uncertainty of approximately 50%. We emphasize that accurate X-ray spectral constraints on gas temperature and metallicity, along with careful modeling of halo mass uncertainty, are essential for accurately estimating the baryon mass for MW-mass galaxies. Future X-ray microcalorimeter missions will be crucial for determining the hot CGM properties and closing the baryon census at the MW-mass scale.