Thermal or Non-thermal? Diffuse emission in the infall region of stacked galaxy groups

Abstract

The faint infall regions surrounding the virial radius of galaxy groups remain largely unexplored due to their low X-ray surface brightness. Using the large statistical power of SRG/eROSITA survey observations, we present the first spectroscopic measurement of the intragroup medium (IGrM) in the infall regions of a large sample of low-mass galaxy groups ($M_{\mathrm{tot}}<1\times 10^{14}{M}_{sun}$), extending to $\sim 2R_{200m}$ (2.2 Mpc). Through spectral stacking of 827 nearby groups from the first eROSITA All-Sky Survey catalog, we detect diffuse emission and measure the thermodynamic properties of gas at densities previously inaccessible to X-ray observations. The stacked spectra are well described by a Gaussian differential emission measure model, yielding a temperature distribution with a mean temperature of $0.96_{-0.04}^{+0.05}$ keV and width of $0.28_{-0.10}^{+0.10}$ keV, and a metal abundance of $0.21_{-0.04}^{+0.06}$ A${}_{sun}$, consistent with expectations for group outskirts. The inferred electron densities decrease from $(4.8\pm 1.3)\times 10^{-5}$cm${}^{-3}$ at $(0.7-2)R_{500c}$ to $(5.5\pm 2.0)\times 10^{-6}$ cm${}^{-3}$ at $(2-4)R_{500c}$, demonstrating eROSITA's ability to probe the low-density outskirts of galaxy groups. Residual emission in the spectra suggests the presence of an additional spectral component. While a secondary thermal interpretation requires an unexpectedly hot, metal-poor plasma, a non-thermal inverse Compton model provides an equally plausible explanation, contributing $\sim 30\%$ of the thermal flux. Assuming that the additional component is produced by inverse Compton emission from a common population of relativistic electrons, the inferred magnetic field strength would be in the sub-$\mu$G regime.

Submitted to A&A, 12 pages, 5 figures, 1 table