The SRG/eROSITA All-Sky Survey

Detection of shock-heated gas beyond the halo boundary into the accretion region

¹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748, Garching, Germany
² Department of Astronomy, University of Maryland, College Park, MD, 20742, USA
³ INAF, Osservatorio di Astrofisica e Scienza dello Spazio, via Piero Gobetti 93/3, I-40129, Bologna, Italy
⁴ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing, 102206, China

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

Received: 26 September 2025
Accepted: 2 March 2026

Abstract

The hot gas in the outskirts of galaxy cluster-sized halos, extending around and beyond the virial radius into nearby accretion regions, remains among one of the least explored baryon components of the large-scale cosmic structure. We present a stacking analysis of 680 galaxy clusters located in the western Galactic hemisphere, using data from the first two years of the Spectrum-Roentgen-Gamma/eROSITA All-Sky Survey. The stacked X-ray surface brightness profile reveals a statistically significant signal extending out to 2 × r_200m (∼4.5 Mpc). The best-fit surface brightness profile is well described by a combination of terms describing orbiting and infalling gas, with a transition occurring around r_200m. At this radius, the best-fit gas number density is 2.5 × 10⁻⁵ cm⁻³, corresponding to a baryon overdensity of 30. By integrating the gas density profile out to r_200m, we inferred a gas fraction higher than the universal baryon fraction with the assumption of a typical halo concentration. However, correcting for possible clumping effects reduces the baryon fraction by more than 20%. Additionally, we examined the distribution of hot gas in massive clusters in the IllustrisTNG simulations, from the halo center to the accretion region. This analysis reveals differences in radial gas profiles depending on whether the direction points toward voids or toward nearby cosmic filaments. Beyond r_200m, the density profile along the filament direction exceeds that along the void direction. This pattern aligns with the observed transition radius between the one-halo and two-halo terms, suggesting that r_200m is the approximate radius marking the location at which cosmic filaments connect to galaxy clusters. Meanwhile, comparisons of the gas density and gas fraction profiles between the observation and the IllustrisTNG simulation suggest that the feedback processes in the stacking sample are more efficient at distributing gas to large radii than the IllustrisTNG model.