An Observationally Motivated Framework for AGN Heating of Cluster Cores

The cooling-flow problem is a long-standing puzzle that has receivedconsiderable recent attention, in part because the mechanism that quenchescooling flows in galaxy clusters is likely to be the same mechanism thatsharply truncates the high end of the galaxy luminosity function. Most of therecent models for halting cooling in clusters have focused on AGN heating, butthe actual heating mechanism has remained mysterious. Here we present aframework for AGN heating derived from a Chandra survey of gas entropy profileswithin cluster cores. This set of observations strongly suggests that the innerparts of cluster cores are shock-heated every ~10^8 years by intermittent AGNoutbursts, driven by a kinetic power output of ~ 10^45 erg/sec and lasting atleast 10^7 years. Beyond ~30 kpc these shocks decay to sound waves, releasingbuoyant bubbles that heat the core's outer parts. Between heating episodes,cooling causes the core to relax toward an asymptotic pure-cooling profile. Thedensity distribution in this asymptotic profile is sufficiently peaked that theAGN shock does not cause a core entropy inversion, allowing the cluster core toretain a strong iron abundance gradient, as observed.Comment: in press, Ap