We present results from high-resolution Tree+SPH simulations of galaxy clusters and groups, aimed at studying the effect of non-gravitational heating on the entropy of the ICM. We simulate three systems, having emission-weighted temperature T=0.6,1 and 3 keV, with spatial resolution better than 1% of the virial radius. We consider the effect of different prescriptions for non-gravitational ICM heating, such as SN energy feedback, as predicted by semi-analytical models of galaxy formation, and two different minimum entropy floors, S_fl=50 and 100 keV cm^2, imposed at z=3. Simulations with only gravitational heating nicely reproduce predictions from self-similar ICM models, while extra heating is shown to break the self-similarity, by a degree which depends on total injected energy and on cluster mass. We use observational results on the excess entropy in central regions of galaxy systems, to constrain the amount of extra-heating required. We find that setting the entropy floor S_fl=50 keV cm^2, which corresponds to an extra heating energy of about 1 keV per particle, is able to reproduce the observed excess of ICM entropy.
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