Feedback Heating by Cosmic Rays in Clusters of Galaxies - AstrophysicsReport as inadecuate

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Abstract: Recent observations show that the cooling flows in the central regions ofgalaxy clusters are highly suppressed. Observed AGN-induced cavities-bubblesare a leading candidate for suppressing cooling, usually via some form ofmechanical heating. At the same time, observed X-ray cavities and synchrotronemission point toward a significant non-thermal particle population. Previousstudies have focused on the dynamical effects of cosmic-ray pressure support,but none have built successful models in which cosmic-ray heating issignificant. Here we investigate a new model of AGN heating, in which theintracluster medium is efficiently heated by cosmic-rays, which are injectedinto the ICM through diffusion or the shredding of the bubbles byRayleigh-Taylor or Kelvin-Helmholtz instabilities. We include thermalconduction as well. Using numerical simulations, we show that the coolingcatastrophe is efficiently suppressed. The cluster quickly relaxes to aquasi-equilibrium state with a highly reduced accretion rate and temperatureand density profiles which match observations. Unlike the conduction-only case,no fine-tuning of the Spitzer conduction suppression factor f is needed. Thecosmic ray pressure, P c-P g <~ 0.1 and dP c-dr <~ 0.1 ho g, is well withinobservational bounds. Cosmic ray heating is a very attractive alternative tomechanical heating, and may become particularly compelling if GLAST detects thegamma-ray signature of cosmic-rays in clusters.

Author: Fulai Guo, S. Peng OH UCSB



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