# Reversing cooling flows with AGN jets: shock waves, rarefaction waves, and trailing outflows

## TL;DR

This paper uses idealized simulations to explore how AGN jets heat galaxy cluster cores through shock waves and outflows, revealing a cyclical process of cooling and heating that impacts cluster evolution.

## Contribution

It provides a detailed physical model of AGN jet interactions with cool-core clusters, highlighting shock, rarefaction, and outflow processes that regulate cooling flows.

## Key findings

- AGN jets drive bow shocks that reverse inflows and heat the core.
- Rarefaction waves transport heated energy outward, preventing immediate cooling.
- Trailing outflows uplifted by AGN bubbles inhibit gas accumulation, leading to cyclical cooling and heating.

## Abstract

The cooling flow problem is one of the central problems in galaxy clusters, and active galactic nucleus (AGN) feedback is considered to play a key role in offsetting cooling. However, how AGN jets heat and suppress cooling flows remains highly debated. Using an idealized simulation of a cool-core cluster, we study the development of central cooling catastrophe and how a subsequent powerful AGN jet event averts cooling flows, with a focus on complex gasdynamical processes involved. We find that the jet drives a bow shock, which reverses cooling inflows and overheats inner cool core regions. The shocked gas moves outward in a rarefaction wave, which rarefies the dense core and adiabatically transports a significant fraction of heated energy to outer regions. As the rarefaction wave propagates away, inflows resume in the cluster core, but a trailing outflow is uplifted by the AGN bubble, preventing gas accumulation and catastrophic cooling in central regions. Inflows and trailing outflows constitute meridional circulations in the cluster core. At later times, trailing outflows fall back to the cluster centre, triggering central cooling catastrophe and potentially a new generation of AGN feedback. We thus envisage a picture of cool cluster cores going through cycles of cooling-induced contraction and AGN-induced expansion. This picture naturally predicts an anti-correlation between the gas fraction (or X-ray luminosity) of cool cores and the central gas entropy, which may be tested by X-ray observations.