AGN Feedback and Bimodality in Cluster Core Entropy

TL;DR

This paper models galaxy clusters to show that AGN feedback can stabilize cool core clusters and explains the observed bimodality in cluster core properties, linking stability to feedback efficiency and thermal conduction.

Contribution

It demonstrates for the first time that AGN feedback can suppress global radial instability in cool core clusters and explains the observed bimodality in cluster core states.

Findings

Global radial instability can be suppressed by AGN feedback above a critical efficiency.

Clusters can be stable as either cool cores with feedback and conduction or non-cool cores with conduction.

Intermediate temperatures lead to unstable solutions, explaining observed bimodality.

Abstract

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) non-cool cores stabilized primarily by conduction. Intermediate central temperatures typically lead to globally unstable solutions. This bimodality is consistent with the recently observed anticorrelation between the flatness of the temperature profiles and the AGN activity (Dunn & Fabian 2008) and the observation by Rafferty et al. (2008) that the shorter central cooling times tend to correspond to significantly younger AGN X-ray cavities.