The SRG/eROSITA All-Sky Survey. Detection of shock-heated gas beyond the halo boundary into the accretion region

TL;DR

The paper collectively advances our understanding of gas in cluster outskirts by stacking 680 eROSITA-detected clusters to detect X-ray emission out to $\sim 2\,r_{200\mathrm{m}}$ ($\sim$4.5 Mpc). It develops a two-component halo model (one-halo gnfw plus a two-halo term) to fit the stacked surface brightness, revealing a gas density of $\sim2.5\times10^{-5}$ cm$^{-3}$ at $r_{200\mathrm{m}}$ and a baryon overdensity of $\sim30$, with a total gas fraction approaching the cosmic value ($\sim90\%$ of $f_b$) by that radius. The analysis identifies a halo–filament connection radius around $r_{200\mathrm{m}}$ and an accretion-shock radius near $\sim3\,r_{200\mathrm{m}}$, consistent with anisotropic gas distributions seen in IllustrisTNG simulations. A comparison indicates that feedback in the observed sample distributes gas more broadly than in the IllustrisTNG model, and the two-halo term likely includes substantial emission from unvirialized gas in filaments and nearby halos; these results motivate future high-sensitivity X-ray missions (e.g., NewAthena) to further map halo boundaries and circumcluster gas.

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 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 SRG/eROSITA All-Sky Survey. The stacked X-ray surface brightness profile reveals a statistically significant signal extending out to 2r200m (~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 r200m. At this radius, the best-fit gas density is 2.5e-5 cm^-3, corresponding to a baryon overdensity of 30. By integrating the gas density profile out to r200m, we infer a gas fraction of 90% of the universal baryon fraction with the assumption of a typical halo concentration, indicating the completeness of the baryon budget within large radii. Additionally, we examine the hot gas distribution 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 probes voids or nearby cosmic filaments. Beyond r200m, 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 r200m is the approximate radius marking the location at which cosmic filaments connect to galaxy clusters. Meanwhile, the comparisons of the gas density profile and gas fraction profile between the observation and the IllustrisTNG simulation suggest that the feedback processes in the stacking sample are more efficient than the IllustrisTNG model in distributing gas to large radii.

Figures (13)

• Figure 1: Spatial distribution of the X-ray emission from the hot gas around a massive dark matter halo. The dashed circle indicates the size of $r_\mathrm{200m}$. At large radii, the halo is connected to and accreting smaller nearby halos from cosmic filaments. This map is produced using gas particles from a $20\times20\times20$ Mpc$^{3}$ box around the id=32 halo in the $z=0$ snapshot of the TNG300-1 simulation (see Sect. \ref{['sect:simulation']} for the details of map creation). The central halo is in a mass $M_\mathrm{500c}=2.8\times10^{14}M_\sun$ and an $r_\mathrm{200m}$ of 2.5 Mpc. Short arrows mark the accretion shock, i.e., the boundary between shock-heated gas and the cool intergalactic medium.
• Figure 2: Top: Mass-redshift distribution of the full eRASS1 galaxy cluster and group sample (gray) from Bulbul2024 and the clusters used in this work (purple). Bottom: Locations of the selected sample on the west Galactic hemisphere eRASS1 X-ray sky.
• Figure 3: The stacked eROSITA surface brightness profile in the 0.2--2.3 keV band after the stray light component has been removed. The radial distance to the cluster center is scaled to the overdensity radius $r_\mathrm{200m}$. The corresponding physical radius given the sample median mass and redshift is labeled at the top of the figure. The top-right inset provides a zoomed-in view of the profile within a zoomed surface brightness range around the background level, with the dashed horizontal line indicating the average surface brightness between 3 and 4 $r_\mathrm{200m}$. The profile shows significant X-ray emission extended to approximately $2\times r_\mathrm{200m}$.
• Figure 4: Best-fit results of the stacked 0.2--2.3 keV eROSITA surface brightness profile, where we adopted the gnfw model for the one-halo component. The width of each component denotes the $1\sigma$ scatter of the posterior samples. The inset shows a zoomed-in view around the background level.
• Figure 5: Best-fit gnfw gas number density profile $1\sigma$ posterior range (purple) and the range of individual fittings of the four wedges (red) inferred by the eROSITA observations. The result from Lyskova2023 (yellow) up to $3\times r_\mathrm{500c}$, and the result from Ghirardini2019 up to $2\times r_\mathrm{500c}$ (green).