THE THREE HUNDRED project: Gas properties outside of galaxy clusters with the WHIM contribution and detection

¹ Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, Anhui 230026, China
² School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
³ Departamento de Física Teórica, M-8, Universidad Autonoma de Madrid, Cantoblanco, 28049 Madrid, Spain
⁴ Centro de Investigación Avanzada en Física Fundamental (CIAFF), Facultad de Ciencias, Universidad Autónoma de Madrid, E-28049 Madrid, Spain
⁵ Institute for Astronomy, University of Edinburgh, Royal Observatory, Edinburgh EH9 3HJ, UK
⁶ Institute for Frontiers in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, China
⁷ Max Planck Institute for Extraterrestrial Physics Giessenbachstrasse 1, 85748 Garching, Germany
⁸ Department of Physics and Astronomy, University of the Western Cape, Robert Sobukwe Rd, Cape Town 7535, South Africa
⁹ School of Physics and Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK

^⋆ Corresponding author: phylrj@mail.ustc.edu.cn

Received: 5 March 2025
Accepted: 1 July 2025

Abstract

We investigate the physical properties and detectability of warm-hot intergalactic medium (WHIM) gas with temperatures in the range 10⁵ < T < 10⁷ K around galaxy clusters using simulated galaxy clusters from THE THREE HUNDRED project. From our simulations with different input physics (GIZMO-SIMBA and GADGET-X), we consistently found that the median gas temperature, as a function of the radial distance from cluster centres, decreases to the WHIM upper bound, ∼10⁷ K, at ∼2 × R_200c. Meanwhile, the WHIM mass fraction increases with radius until ∼3 × R_200c, where it plateaus slightly above 70% for both simulations at all investigated redshifts. By simulating X-ray emissions from all gas components, we found that the WHIM contribution at the soft X-ray band (0.2–2.3 keV) also increases with radius but eventually plateaus at larger distances. The differences between the two simulations become more pronounced at higher redshifts and larger radii, indicating that the WHIM observation can be used to constrain baryon models. After accounting for observational effects, primarily by removing (sub)halos, we predict that the signal-to-noise ratio of the X-ray signal obtained by stacking the eRASS1 galaxy cluster catalogue will be ∼6.4 for GIZMO-SIMBA and ∼20.8 for GADGET-X.