Entropy "floor'' and effervescent heating of intracluster gas

Recent X-ray observations of clusters of galaxies have shown that the entropy of the intracluster medium (ICM), even at radii as large as half the virial radius, is higher than that expected from gravitational processes alone. This is thought to be the result of nongravitational processes influencing the physical state of the ICM. In this paper we investigate whether heating by central active galactic nuclei (AGNs) can explain the distribution of excess entropy as a function of radius. The AGNs are assumed to inject buoyant bubbles into the ICM, which heat the ambient medium by doing pdV work as they rise and expand. Several authors have suggested that this "effervescent heating'' mechanism could allow the central regions of clusters to avoid the "cooling catastrophe.'' Here we study the effect of effervescent heating at large radii. We find that the results are mainly sensitive to the total energy injected into the cluster. Our calculations show that such a heating mechanism is able to solve the entropy problem, provided that the total energy injected by the AGN is roughly proportional to the cluster mass. The inferred correlation is consistent with a linear relation between the mass of the central black hole(s) and the mass of the cluster, which is reminiscent of the Magorrian relation between the black hole and bulge mass.