Suppressing cluster cooling flows by self-regulated heating from spatially distributed population of AGNs

Abstract

(Abridged) Existing models invoking AGN activty to resolve the cooling flow conundrum in galaxy clusters focus exclusively on the role of the central galaxy. Such models require fine-tuning of highly uncertain microscopic transport properties to distribute the thermal thermal over the entire cluster cooling core. We propose that the ICM is instead heated by multiple, spatially distributed AGNs. There is mounting observational evidence for multiple AGNs in cluster environments. Active AGNs drive bubbles into the ICM. We identify three distinct interactions between the bubble and the ICM: (1) Upon injection, the bubbles expand rapidly in situ to reach pressure equilibrium with their surroundings, generating shocks and waves whose dissipation is the principal source of ICM heating. (2) Once inflated, the bubbles rise buoyantly at rate determined by a balance with the viscous drag force, which itself results in some additional heating. (3) Rising bubbles expand and compress their surroundings. This process is adiabatic and does not contribute to any additional heating; rather, the increased ICM density due to compression enhances cooling. Our model sidesteps the ``transport'' issue by relying on the spatially distributed galaxies to heat the cluster core. We include self regulation in our model by linking AGN activity in a galaxy to cooling characteristics of the surrounding ICM. We use a spherically symmetric one-dimensional hydrodynamical code to carry out a preliminary study illustrating the efficacy of the model. Our self-regulating scenario predicts that there should be enhanced AGN activity of galaxies inside the cooling regions compared to galaxies in the outer parts of the cluster. This prediction remains to be confirmed or refuted by observations.