Preheating the Intracluster Medium in High-Resolution Simulations: The Effect on the Gas Entropy

We present first results from high-resolution tree + smoothed particle hydrodynamics simulations of galaxy clusters and groups that are aimed at studying the effect of nongravitational heating on the entropy of the intracluster medium (ICM). We simulate three systems having emission-weighted temperatures Tew ≃ 0.6, 1, and 3 keV with spatial resolutions better than 1% of the virial radius. We consider the effect of different prescriptions for nongravitational ICM heating, such as supernova energy feedback, as predicted by semianalytical models of galaxy formation, and two different minimum entropy floors, Sfl = 50 and 100 keV cm2, 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 that depends on the total injected energy and on the cluster mass. We use the observational results on the excess entropy in central regions of galaxy systems to constrain the amount of extra heating required. We find that by setting the entropy floor Sfl = 50 keV cm2, which corresponds to an extra heating energy of about 1 keV per particle, we are able to reproduce the observed excess of ICM entropy.