ABSTRACT
We review and discuss the processes that govern the amount and the thermal state of the hot plasma pervading clusters and groups of galaxies: the gravitational heating driven by the merging histories of the Dark Matter (DM), the radiative cooling of the baryons, and the energy fed back to them by SNe and by AGNs or quasars. We show that the x-ray emissions, the entropy levels, and the entropy profiles observed throughout the range from clusters to groups require the AGNs to contribute substantially to preheat the plasma and hinder its gathering in the poor clusters. The observations also require the quasars to blow some of the plasma out of groups and galaxies; this process can be caught in the act by resolved observations of enhanced Sunyaev-Zel’dovich signals.
