The impact of assembly history on the X-ray detectability of halos

From galaxy groups to galaxy 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 INAF – Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁶ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, I-34014 Trieste, Italy
⁷ INFN, Sezione di Trieste, Via Valerio 2, 34127 Trieste, TS, Italy
⁸ ICSC - Centro Nazionale di Ricerca in High Performance Computing, Big Data e Quantum Computing, Via Magnanelli 2, Bologna, Italy
⁹ Sorbonne Université, UMR7095, Institut d’Astrophysique de Paris, 98 bis Boulevard Arago, F-75014 Paris, France
¹⁰ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
¹¹ International Centre for Radio Astronomy Research, University of Western Australia, M468, 35 Stirling Highway, Perth, WA, 6009 Australia
¹² Department of Physics, College of Natural and Mathematical Sciences, University of Dodoma, PO Box 338 Dodoma, Tanzania
¹³ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
¹⁴ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, (LC), Italy
¹⁵ Como Lake Center for Astrophysics (CLAP), DiSAT, Università degli Studi dell’Insubria, Via Valleggio 11, I-22100 Como, Italy

^⋆ Corresponding author: ilaria.marini@eso.org

Received: 21 March 2025
Accepted: 17 April 2025

Abstract

Galaxy groups represent a significant fraction of the halo population, playing a crucial role in galaxy formation and evolution. However, their detection in X-rays remains challenging, raising questions about the physical mechanisms driving their detectability in current surveys. Using the Magneticum simulations, we construct a mock X-ray lightcone of the local Universe (z < 0.2) to investigate the selection function of galaxy groups and clusters. We find that the central supermassive black holes (SMBH) activity is a key driver of baryon depletion, but late-time mergers boost X-ray brightness by replenishing the gas reservoir in the halos, highlighting the interplay between feedback processes and the environment. Our analysis shows that X-ray bright groups experience sustained late-time mass accretion, maintaining higher gas fractions and fuelling the central SMBH, further increasing the X-ray emissivity in the core. In contrast, X-ray faint groups form earlier and lose most of their gas over time, resembling fossil groups. These faint groups reside preferentially in under-dense regions, unable to re-accrete efficiently baryons at later times. Magneticum predicts strong anti-correlations between gas fraction (or X-ray luminosity) and SMBH mass, stellar mass (both in the central galaxy and intracluster light), and group richness at fixed halo mass. We derive predictions on the hot gas fraction at fixed halos mass (e.g. a group of total mass M₅₀₀ = 10¹³ M_⊙ can have hot gas fractions in the range f_gas = 0.02 − 0.06 and a central SMBH with a median mass of M_BH = 10⁹ M_⊙ and a scatter of 0.5 dex) compatible with the most recent measurements of the baryonic fraction. These findings will aid the interpretation of future X-ray surveys, demonstrating the power of simulation-based inference.