The entropy distribution in clusters: evidence of feedback?

The entropy of the intracluster medium at large radii has been shown recently to deviate from the self-similar scaling with temperature. Using N-body/hydrodynamic simulations of the LCDM cosmology, we demonstrate that this deviation is evidence that feedback processes are important in generating excess entropy in clusters. While radiative cooling increases the entropy of intracluster gas, resulting in a good match to the data in the centres of clusters, it produces an entropy-temperature relation closer to the self-similar scaling at larger radii. A model that includes feedback from galaxies, however, not only stabilises the cooling rate in the simulation, but is capable of reproducing the observed scaling behaviour both in cluster cores and at large radii. Feedback modifies the entropy distribution in clusters due to its increasing ability at expelling gas from haloes with decreasing mass. The strength of the feedback required, as suggested from our simulations, is consistent with supernova energetics, providing a large fraction of the energy reaches low-density regions and is originally contained within a small mass of gas.