Heating of the Hot Intergalactic Medium by Powerful Radio Galaxies and Associated High‐Energy Gamma‐Ray Emission

Abstract

There is increasing evidence that some heating mechanism in addition to gravitational shock heating has been important for the hot gas inside clusters and groups of galaxies, as indicated by their observed X-ray scaling properties. While supernovae are the most obvious candidate heating sources, a number of recent studies have suggested that they may be energetically insufficient. Here we consider high-power, FR II radio galaxies and shock heating of the intracluster medium (ICM; including the case of the intergalactic medium prior to cluster formation) by their large-scale jets. Based on the observed statistics of radio galaxies in clusters and their evolution, along with the most reasonable assumptions, it is shown that they can provide the ICM with excess specific energies of 1-2 keV particle-1, mainly during the redshift interval z ~ 1-3. This naturally meets the requirements of cluster evolution models with nongravitational feedback in accounting for the observed deviations in the X-ray luminosity-temperature relation. In contrast to supernovae, such large-scale jets deposit their energy directly into the low-density ICM outside galaxies and are much less susceptible to radiative losses. As a clear and potentially decisive test of this scenario, we propose the observation of prompt high-energy gamma rays emitted by shock-accelerated, nonthermal electrons during the epoch of ICM heating by radio galaxies, which may be feasible with the GLAST satellite. Implications for recent detections of excess hard X-rays from groups are also discussed.