Physical implications of the X-ray properties of galaxy groups and clusters

Within the standard framework of structure formation, where clusters and groups of galaxies are built up from the merging of smaller systems, the physical properties of the intracluster medium, such as the gas temperature and the total X-ray luminosity, are predicted to possess well-defined, self-similar scaling relations. Observed clusters and groups, however, show strong deviations from these predicted relations. We argue that these deviations are unlikely to be entirely due to observational biases; we assume they are physically based, due to the presence of excess entropy in the intracluster medium in addition to that generated by accretion shocks during the formation of the cluster. Several mechanisms have been suggested as a means of generating this entropy. Focusing on those mechanisms that pre-heat the gas before it becomes a constituent of the virialized cluster environment, we present a simple, intuitive, physically motivated, analytic model that successfully captures the important physics associated with the accretion of high-entropy gas on to group- and cluster-scale systems. We use the model to derive the new relationships between the observable properties of clusters and groups of galaxies, as well as the evolution of these relations. These include the luminosity–temperature and luminosity–σ relations, as well as the temperature distribution function and X-ray luminosity function. These properties are found to be a more accurate description of the observations than those predicted from the standard framework. Future observations that will further test the efficacy of the pre-heated gas scenario are also discussed.