Average X-ray properties of galaxy groups: From Milky Way-like halos 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 Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁷ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁸ International Centre for Radio Astronomy Research, University of Western Australia, M468, 35 Stirling Highway, Perth, WA 6009, Australia
⁹ McMaster University, 1280 Main Street West, Hamilton, Ontario L8S 4L8, Canada
¹⁰ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, I-34014 Trieste, Italy
¹¹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, (LC), Italy
¹² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹³ INAF – Osservatorio Astronomico di Bologna, Via Gobetti 93/3, 40129 Bologna, Italy
¹⁴ Max-Planck-Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, D-85748 Garching bei München, Germany

^★ Corresponding author: paola.popesso@eso.org

Received: 2 December 2024
Accepted: 24 June 2025

Abstract

Context. In this study, we present the average X-ray properties of massive halos at z < 0.2 over the largest halo mass range ever probed so far, bridging the gap from Milky Way-like halos to massive clusters.

Aims. The results show the average X-ray properties of galaxy groups, obtained through the stacking analysis in the eFEDS area of the GAMA galaxy group sample at z < 0.2. The results have been rigorously tested using a synthetic dataset that mirrors the observed eROSITA X-ray and GAMA optical data based on the lightcones of the Magneticum simulations.

Methods. We used a halo mass proxy based on group total luminosity, avoiding systematics linked to velocity dispersion and richness cuts. The stacking is done in bins of halo mass and tested in the synthetic dataset for AGN and X-ray binaries contamination, systematics due to the halo mass proxy, and uncertainty in the optical group center.

Results. We provide the average X-ray surface brightness profile in six bins of mass, ranging from Milky Way-like systems to poor clusters at M₂₀₀ ∼ 10¹⁴ M_⊙. We find that the scatter in the L_X − M relation is driven by gas concentration in groups, as undetected X-ray systems at fixed halo mass exhibit lower central gas concentrations than detected ones, aligning with Magneticum predictions. However, there is a discrepancy regarding dark matter concentration: Magneticum predictions suggest that undetected groups are more concentrated, implying they are older and more relaxed, whereas previous observational findings suggest the opposite. We present new measurements of the L_X, 500 − M₅₀₀ and L_X, 200 − M₂₀₀ relations, from Milky Way-like halos to massive clusters. Our results indicate that a single power law fits the data across three decades of halo mass, and they align well with previous studies focused on specific halo mass ranges. Magneticum best matches the observed gas distribution across the entire halo mass range, while IllustrisTNG, EAGLE, Simba, and FLAMINGO show larger discrepancies at different mass ranges. This highlights that simulations such as Magneticum, which are not calibrated on z = 0 galaxy properties, reproduce gas properties well but still lead to overly massive galaxies at the centers of massive halos. Conversely, simulations calibrated on z = 0 galaxy properties fail to reproduce the gas properties.

Conclusions. This evidence reveals a potential gap in our understanding of the relationship between galaxies and their host structures. Therefore, this work emphasizes the need for a deeper investigation into the connection between gas and dark matter distributions and their impact on central galaxy properties. Such an inquiry is crucial to comprehensively understanding the role and interplay of gravitational forces and feedback-related processes in shaping both the large-scale structure and the galaxy population.