We investigate how the properties of the dark matter and baryons in groups and clusters of galaxies depend on the mass of such structures. While the density profiles of dark matter are approximatively self-similax (i.e., their shape do not depend on the mass scale), those of the gas (observable through its copious X-ray emission) become shallower for masses below similar to 5 x 10(13) Mcircle dot.
We show how such observed properties point toward pre-heating of the gas in smaller condensations before its inclusion in groups or clusters, strongly supporting the hierarchical clustering scenario for the formation of cosmic structures. Extra-heating from Supernovae following star formation in the galaxies which built up the groups/clusters can account for the observations in a Lambda- dominated or in a critical Universe, while they fail to provide the observed X-ray properties of galaxy groups in open Universes or in models with a high fraction of hot dark matter (Omega(hot) > 0.2).
