ar X iv : a st ro - p h / 99 12 18 4 v 1 9 D ec 1 99 9 The Virtues of X – ray Clusters and the Entropy of Cos - mic

The thermodynamics of the diffuse, X-ray emitting gas in clusters and groups of galaxies are affected by a certain amount of non–gravitational energy input, as indicated by the scaling properties of X–ray halos. Such a view has been recently confirmed by the detection of an excess entropy in the center of groups. It is not easy, however, to identify unambiguously the source and the epoch of such excess entropy. Here we show that the observations of high z clusters will help in reconstructing both the dynamic and the thermodynamic history of the diffuse cosmic baryons.