The hot circumgalactic media of massive cluster satellites in the TNG-Cluster simulation: existence and detectability
The most massive galaxy clusters in the Universe host tens to hundreds of massive satellite galaxies, but it is unclear if these satellites are able to retain their own gaseous atmospheres. We analyze the evolution of $\sim90,000$ satellites of stellar mass $\sim10^{9-12.5}\,M_\odot$ around 352 galaxy clusters of mass $M_{\rm vir}\sim10^{14.3-15.4}\,M_\odot$ at $z=0$ from the new TNG-Cluster suite of cosmological magneto-hydrodynamical galaxy cluster simulations. The number of satellites per host increases with host mass, and the mass--richness relation broadly agrees with observations. A halo of mass $M_{\rm 200c}\sim10^{14.5}\,(10^{15})\,M_\odot$ hosts $\sim100\,(300)$ satellites today. Only a minority of satellites retain some gas, and this fraction increases with stellar mass. Lower mass satellites $\sim10^{9-10}\,M_\odot$ are more likely to retain part of their cold interstellar medium, consistent with ram pressure preferentially removing hot extended gas first. At higher stellar masses $\sim10^{10.5-12.5}\,M_\odot$ the fraction of gas-rich satellites increases to unity, and nearly all satellites retain a sizeable portion of their hot, spatially-extended circumgalactic medium (CGM), despite the ejective activity of their supermassive black holes. According to TNG-Cluster, the CGM of these gaseous satellites can be seen in soft X-ray emission (0.5-2.0 keV) that is $\gtrsim10$ times brighter than the local background. This X-ray surface brightness excess around satellites extends to $\sim30-100$ kpc, and is strongest for galaxies with higher stellar masses and larger host-centric distances. Approximately 10 per cent of the soft X-ray emission in cluster outskirts $\sim0.75-1.5R_{\rm 200c}$ originates from satellites. The CGM of member galaxies reflects the dynamics of cluster-satellite interactions and contributes to the observationally-inferred properties of the intracluster medium.