Could AGN Outbursts Transform Cool Core Clusters?

TL;DR

This paper explores how AGN outbursts and variable thermal conduction could cause galaxy clusters to cycle between cool-core and non cool-core states, explaining observed bimodality.

Contribution

It proposes a model where AGN activity and conduction variability drive the CC/NCC state transitions, offering a potential explanation for cluster bimodality.

Findings

AGN heating can transform CC clusters into NCC states.

Conductive heating maintains NCC states in clusters.

Bimodality is predicted to diminish in galaxy groups.

Abstract

The origin of the bimodality in cluster core entropy is still unknown. At the same time, recent work has shown that thermal conduction in clusters is likely a time-variable phenomenon. We consider if time-variable conduction and AGN outbursts could be responsible for the cool-core (CC), non cool-core (NCC) dichotomy. We show that strong AGN heating can bring a CC cluster to a NCC state, which can be stably maintained by conductive heating from the cluster outskirts. On the other hand, if conduction is shut off by the heat-flux driven buoyancy instability, then the cluster will cool to the CC state again, where it is stabilized by low-level AGN heating. Thus, the cluster cycles between CC and NCC states. In contrast with massive clusters, we predict the CC/NCC bimodality should vanish in groups, due to the lesser role of conductive heating there. We find tentative support from the distribution of central entropy in groups, though firm conclusions require a larger sample carefully controlled for selection effects.