Constraints on the Hot Circumgalactic Medium Around Nearby L * Galaxies from the SRG/eROSITA All Sky Survey

Abstract The circumgalactic medium (CGM), a multiphase gas acting as the dynamic interface between the main body of a normal galaxy and the intergalactic medium, is a key ingredient of the galactic ecosystem and provides crucial diagnostics to a wide array of physical processes affecting galaxy evolution. However, direct evidence for a predominantly hot (at a characteristic temperature of a few million kelvins) CGM around present-day L * galaxies remains elusive, despite extensive observational searches and strong physical reasons that it should exist. Here, we present a systematic search for the hot CGM around a representative sample of nearby L * galaxies selected from the 50 Mpc Galaxy Catalog (50MGC) by stacking their X-ray images and spectra from the Spectrum-Roentgen-Gamma (SRG)/eROSITA all-sky survey. Significant (7.4 σ ) diffuse X-ray emission is detected out to a galactocentric radius ≳50 kpc, the cumulative spectrum of which requires a substantial thermal component, consistent with a hot plasma with log-normal temperature distribution, and thus argues against a predominantly nonthermal origin. The diffuse emission exhibits a relatively steep intensity profile ( β = 0.5 2 − 0.06 + 0.08 ), suggesting a moderate amount (2 × 10 10 M ⊙ ) of hot gas within 10–200 kpc. The radial distribution and total amount of hot CGM gas are broadly in agreement with predictions by IllustrisTNG simulations for present-day Milky Way like galaxies, while also showing dependencies on both star formation and nuclear activity. The constraints on the hot CGM derived in this study hold promise for calibrating key physical processes, such as stellar feedback and active galactic nucleus feedback, in next-generation cosmological simulations.