A GLOBAL STABILITY ANALYSIS OF CLUSTERS OF GALAXIES WITH CONDUCTION AND AGN FEEDBACK HEATING

We investigate a series of steady state models of galaxy clusters, in which the hot intracluster gas is efficiently heated by active galactic nucleus (AGN) feedback and thermal conduction, and in which the mass accretion rates are highly reduced compared to those predicted by the standard cooling flow models. We perform a global Lagrangian stability analysis. We show for the first time that the global radial instability in cool core clusters can be suppressed by the AGN feedback mechanism, provided that the feedback efficiency exceeds a critical lower limit. Furthermore, our analysis naturally shows that the clusters can exist in two distinct forms. Globally stable clusters are expected to have either (1) cool cores stabilized by both AGN feedback and conduction or ( 2) noncool cores stabilized primarily by conduction. Intermediate central temperatures typically lead to globally unstable solutions. This bimodality is consistent with the recent observation by Dunn & Fabian of anticorrelation between the flatness of the temperature profiles and the AGN activity and the observation by Rafferty et al. that the shorter central cooling times tend to correspond to significantly younger AGN X-ray cavities.