X-ray emission in IllustrisTNG circum-cluster environments

II. Possible origins of the soft X-ray excess emission

¹ Sorbonne Université, UMR7095, Institut d’Astrophysique de Paris 98 bis Boulevard Arago 75014 Paris, France
² Max-Planck Institute for Astrophysics Karl-Schwarzschild-Str. 1 D-85741 Garching, Germany
³ Kavli IPMU (WPI), UTIAS, The University of Tokyo Kashiwa Chiba 277-8583, Japan
⁴ University of Alabama in Huntsville, Department of Physics and Astronomy Huntsville AL 35899, USA
⁵ Instituto de Astrofísica de Andalucía, Consejo Superior de Investigaciones Científicas (CSIC) Glorieta de la Astronomía s/n E-18008 Granada, Spain
⁶ New York City College of Technology, City University of New York, Physics Department Brooklyn NY, USA
⁷ Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale 91405 Orsay, France
⁸ Laboratoire de Physique de l’Ecole Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité F-75005 Paris, France

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

Received: 23 June 2025
Accepted: 10 January 2026

Abstract

Context. An excess of soft X–ray emission (∼0.2 − 1 keV) above the contribution from the hot intracluster medium (ICM) has been detected in a number of galaxy clusters, including the Coma cluster. The physical origin of this emitting medium above the hot ICM has not yet been determined, in particular, it is unclear whether it is thermal or nonthermal.

Aims. We investigate the gas phase and gas structure that reproduce the soft excess radiation from the cluster core to the outskirts best using simulations.

Method. By using the IllustrisTNG simulation (TNG300), we predict the radial profile of thermodynamic properties and the soft X–ray surface brightness of 138 clusters within 5 × r₂₀₀. Their X–ray emission was simulated for the hot ICM gas phase (T ≥ 10⁷ K), the entire warm–hot medium at a temperature T = 10^5 − 7K (WARM), and for the diffuse and low–density warm–hot intergalactic medium (WHIM).

Results. The soft excess inside clusters appears to be produced by substructures of the WARM gas phase that host dense warm clumps, that is, the warm circumgalactic medium (WCGM), and the inner soft excess is strongly correlated with substructure and the WCGM mass fractions. Outside of the virial radius, the fraction of WHIM gas that is mostly inside filaments that are connected to clusters boosts the soft X–ray excess. The more diffuse the gas, the higher the soft X-ray excess beyond the virial region.

Conclusion. The thermal emission of the WARM gas phase in the form of WCGM clumps and WHIM diffuse filaments reproduces the soft excess emission that was observed up to the virial radius in Coma and in the inner regions of other massive clusters. Moreover, our analysis suggests that soft X–ray excess is a proxy of the dynamical cluster state and that higher excess is observed in the most unrelaxed clusters.