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

¹ European Southern Observatory, Karl Schwarzschildstrasse 2, 85748 Garching bei München, Germany
² Excellence Cluster ORIGINS, Boltzmannstr. 2, D-85748 Garching bei München, Germany
³ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr.1, 81679 München, Germany
⁴ Max-Planck-Institut für Astrophysik, Karl-Schwarzschildstr. 1, 85741 Garching bei München, Germany
⁵ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁶ Shanghai Astronomical Observatory (SHAO) at the Chinese Academy of Sciences, 80 Nandan Road, Xuhui District, Shanghai 200030, China
⁷ Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁸ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, Via A. Corti 12, 20133 Milano, Italy
⁹ INFN, Sezione di Bologna, Viale Berti Pichat 6/2, 40127 Bologna, Italy
¹⁰ INAF, Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy
¹¹ INAF, Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
¹² IFPU, Institute for Fundamental Physics of the Universe, Via Beirut 2, I-34014 Trieste, Italy
¹³ International Centre for Radio Astronomy Research, University of Western Australia, M468, 35 Stirling Highway, Perth, WA 6009, Australia
¹⁴ Max-Planck-Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, D-85748 Garching bei München, Germany
¹⁵ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹⁶ Como Lake Center for Astrophysics (CLAP), DiSAT, Università degli Studi dell’Insubria, Via Valleggio 11, I-22100 Como, Italy
¹⁷ Physics Department, College of Natural and Mathematical Sciences, P.O.Box 338 The University of Dodoma, Tanzania

^⋆ Corresponding author: victoria.toptun@eso.org

Received: 3 March 2025
Accepted: 1 June 2025

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 10¹³ to 10¹⁴ 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 (10¹³ 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: log₁₀(M₅₀₀/M_⊙) = (1.65 ± 0.11)⋅log₁₀(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 MAGNETICUM 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.