The Heating of the ICM: Energy Crisis and viable solutions

X–ray observations indicate that non–gravitational processes play a key role in the thermodynamics of the Intra Cluster Medium (ICM). The effect of non–gravitational processes is imprinted in the ICM as an entropy minimum, whose effects are visible in the Luminosity– Temperature relation and in the Entropy–Temperature relation. However, the X–ray emission alone cannot discriminate between different mechanisms and sources of heating. There are no answers at present to the following questions: how much non–gravitational energy per baryons is present in the ICM? When was this energy injected? Which are the sources of heating? The embarrassment in front of these questions is amplified by the fact that the most viable sources of heating, SNae and stellar winds, seem to be inefficient in bringing the ICM to the observed entropy level. We may call it the energy crisis. Here we review the main aspects of this crisis, listing possible solutions, including other sources, like AGNs and Radio Galaxies, or other mechanisms, like large scale shocks and selective cooling. 1. Evidences for non–gravitational heating Observations in the X–ray band provided convincing evidences for non–gravitational heating of the diffuse baryons in the potential wells of groups and clusters of galaxies (ICM). Among them: the shape of the L–T relation (steeper than the self–similar behaviour L ∝ T 2 predicted in the case of gravitational processes only) and the entropy excess in the center of groups, recently found by Ponman, Cannon & Navarro (1999). Non–gravitational heating of the ICM is expected also on the basis of observations of an average metallicity Z ≃ 0.3 Z⊙. In fact, SNae and stellar winds are the most viable source of heating and metal enrichment of the ICM. It should be understood how much of the ejected energy goes into the ICM and if it is enough to generate the observed entropy plateau. 2. How the entropy works If the excess entropy is present in the baryons before collapse (external heating, or preheating, see Tozzi & Norman 2001, hereafter TN), it will be preserved in the cores of dark matter halos after virialization. In virtue of the extra pressure support, in fact, the gas is accreted adiabatically without shock heating. The excess entropy also decreases the density in the central regions. This, in turn,