The hot gas mass fraction in halos

From Milky Way-like 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 INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁴ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, I-34014 Trieste, Italy
⁵ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr.1, 81679 München, Germany
⁶ Max-Planck-Institut für Astrophysik, Karl-Schwarzschildstr. 1, 85741 Garching bei München, Germany
⁷ Universität Innsbruck, Institut für Astro- und Teilchenphysik, Technikerstr. 25/8, 6020 Innsbruck, Austria
⁸ INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁰ International Centre for Radio Astronomy Research, University of Western Australia, M468, 35 Stirling Highway, Perth, WA 6009, Australia
¹¹ McMaster University, 1280 Main Street West Hamilton, Ontario L8S 4L8, Canada
¹² INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Milano, via A. Corti 12, 20133 Milano, Italy
¹³ Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
¹⁴ INAF – Osservatorio Astronomico di Bologna, Via Gobetti 93/3, 40129, Bologna, Italy
¹⁵ Max-Planck-Institut für Extraterrestrische Physik (MPE), Giessenbachstr. 1, D-85748 Garching bei München, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 December 2024
Accepted: 6 August 2025

Abstract

Aims. By using eROSITA data in the eFEDS area, we provide a measure of the f_gas − M_halo relation over the largest halo mass range, from Milky Way-sized halos to massive clusters, and to the largest radii (R₂₀₀) ever probed so far in local systems at z < 0.2.

Methods. To cope with incompleteness and selection biases of the X-ray selection, we applied the stacking technique in eROSITA data of a highly complete and tested sample of optically selected groups. The method has been extensively tested on mock observations.

Results. In massive clusters, the hot gas alone provides a baryon budget within R₂₀₀ consistent with Ω_b/Ω_m, while at the group mass scale, it accounts only for 20–40% of the cosmic value. The f_gas − M_halo relation is well fit by a power law with a consistent slope (within 1σ) at R₅₀₀ and R₂₀₀ and a normalization varying nearly by a factor of two. Such a relation is consistent with other works in the literature that consider X-ray survey data at the same depth as eFEDS, but it provides a lower average f_gas in the group regime in comparison to works based on X-ray bright group samples. Comparison of the observed relation with the predictions of several hydrodynamical simulations (BAHAMAS, FLAMINGO, SIMBA, Illustris, IllustrisTNG, MillenniumTNG, and Magneticum) shows that all state-of-the-art simulations except Magneticum overpredict the gas fraction, with the largest discrepancy (up to a factor three) being in the 10^13.5 − 10^14.5 M_⊙ halo mass range.

Conclusions. We emphasize the need for mechanisms that can effectively expel gas to larger radii in galaxy groups without excessively quenching star formation in their member galaxies. Current hydrodynamical simulations face a significant challenge in balancing their subgrid physics, as none can sufficiently evacuate gas from the halo virial region without negatively impacting the properties of the resident galaxy population.