# X-ray surface brightness and gas density profiles of galaxy clusters up to 3*R500c with SRG/eROSITA

**Authors:** N. Lyskova, E. Churazov, I. I. Khabibullin, R. Burenin, A. A. Starobinsky, R. Sunyaev  
**arXiv:** [2305.07080](https://arxiv.org/abs/2305.07080)  
**Date:** 2023-05-11  
**Summary page:** https://www.summarizepaper.com/en/paper/2305.07080v1/

## Key points

- Study conducted using data from the SRG/eROSITA all-sky survey
- Analyzed X-ray surface brightness and gas density profiles of galaxy clusters
- Stacked approximately 40 galaxy cluster images in the 0.3-2.3 keV band
- Excess emission detected beyond 3 times R_500c, indicating presence of gas in these regions
- Reconstructed density profile showed good agreement with mean gas profile predicted by simulations (within about 30%)
- Simulations suggest local gas overdensity of around 20-30 at 3 times R_500c and gas fraction close to Omega_b/Omega_m ≈ 0.15 in standard ΛCDM model
- Gas temperature decreases by a factor of approximately 4-5 beyond 3 times R_500c compared to typical cluster temperatures within R_500c
- Entropy continues to increase up to this distance despite decrease in temperature
- Larger sample size needed for more precise constraints on gas properties beyond 3 times R_500c

## Summary

Using data from the SRG/eROSITA all-sky survey, a study was conducted to analyze the X-ray surface brightness and gas density profiles of galaxy clusters. The researchers stacked approximately 40 galaxy cluster images in the 0.3-2.3 keV band, covering a radial range of up to 10 times the critical radius ($R_{\rm 500c}$). The analysis revealed that there is excess emission detected beyond $3\times R_{\rm 500c}$, indicating the presence of gas in these regions. Furthermore, the reconstructed density profile obtained from the X-ray surface brightness profile showed good agreement (within about 30%) with the mean gas profile predicted by numerical simulations. These simulations suggest a local gas overdensity of around 20-30 at $3\times R_{\rm 500c}$ and a gas fraction close to $\frac{\Omega_b}{\Omega_m}\approx 0.15$ in the standard $\Lambda$CDM model. Comparing the derived gas density profile with electron pressure profiles based on Sunyaev-Zel'dovich (SZ) measurements, it was observed that beyond $3\times R_{\rm 500c}$, the gas temperature decreases by a factor of approximately 4-5 compared to typical cluster temperatures within $R_{\rm 500c}$. However, despite this decrease in temperature, entropy continues to increase up to this distance. To obtain more precise constraints on gas properties beyond $3\times R_{\rm 500c}$, a larger sample size than what was used in this pilot study is needed. Overall, this study provides valuable insights into the X-ray surface brightness and gas density profiles of galaxy clusters, shedding light on their properties and behavior at large radial distances.