The eROSITA view on the halo mass–temperature relation: From low-mass groups to massive clusters

Abstract

Galaxy groups and clusters are among the best probes of structure formation and growth in a cosmological context. Most of their baryonic component is dominated by hot plasma, known as the intracluster medium (ICM) in clusters or the intragroup medium (IGrM) in groups. Their thermodynamical properties serve as indicators of the halo’s dynamical state and can be used to determine halo mass in the self-similar scenario. However, baryonic processes, such as AGN feedback and gas cooling, may affect the global properties of the ICM, especially in the group regime. These effects might lead to deviations from self-similar predictions in the scaling relations of galaxy groups, while they remain in place for massive galaxy clusters. Additionally, the low-mass end of the scaling relations, ranging from 10 13 to 10 14 M ⊙ , remains unclear and poorly populated, as current X-ray surveys detect only the brightest groups. Here, we present the mass-temperature relation across the entire mass range, from massive clusters to low-mass groups (10 13 M ⊙ ), as observed by eROSITA. Using spectral stacking from the first eROSITA All-Sky Survey data for optically selected galaxy groups, we find that, in the lower mass range, galaxy groups follow the power-law relation known for galaxy clusters. We provide the best-fit mass–temperature relation, validated over two decades in halo mass, as follows: log 10 ( M 500 / M ⊙ ) = (1.65 ± 0.11)⋅log 10 ( T X /1 keV)+(13.38 ± 0.05). We further validate these results by conducting the same stacking procedure on mock eRASS:4 data using the M AGNETICUM hydrodynamical simulation. This indicates that AGN feedback is more likely to affect the distribution of baryons in the intragroup medium rather than the overall halo gas temperature. No significant changes in the slope of the mass-temperature relation suggest that temperature can serve as a reliable mass proxy across the entire mass range. This supports the use of temperature-derived masses, particularly in cosmological studies, significantly broadening the mass range and enabling applications such as improving cluster mass function studies and cosmological parameter estimates.