Mechanical Feedback from Active Galactic Nuclei in Galaxies, Groups, and Clusters

TL;DR

This paper reviews how active galactic nuclei regulate cooling and star formation in galaxies, groups, and clusters through mechanical feedback, highlighting the self-regulated nature of this process and its implications for galaxy evolution.

Contribution

It discusses the coupling of cooling atmospheres with feedback mechanisms, emphasizing the role of black hole activity and outflows in galaxy and cluster evolution.

Findings

Cooling is offset by mechanical heating from AGN radio jets.

Outflows disperse elements on large scales from central galaxies.

Evidence suggests mechanical heating influences hot halo entropy and cooling flow evolution.

Abstract

The radiative cooling timescales at the centers of hot atmospheres surrounding elliptical galaxies, groups, and clusters are much shorter than their ages. Therefore, hot atmospheres are expected to cool and to form stars. Cold gas and star formation are observed in central cluster galaxies but at levels below those expected from an unimpeded cooling flow. X-ray observations have shown that wholesale cooling is being offset by mechanical heating from radio active galactic nuclei. Feedback is widely considered to be an important and perhaps unavoidable consequence of the evolution of galaxies and supermassive black holes. We show that cooling X-ray atmospheres and the ensuing star formation and nuclear activity are probably coupled to a self-regulated feedback loop. While the energetics are now reasonably well understood, other aspects of feedback are not. We highlight the problems of atmospheric heating and transport processes, accretion, and nuclear activity, and we discuss the potential role of black hole spin. We discuss X-ray imagery showing that the chemical elements produced by central galaxies are being dispersed on large scales by outflows launched from the vicinity of supermassive black holes. Finally, we comment on the growing evidence for mechanical heating of distant cluster atmospheres by radio jets and its potential consequences for the excess entropy in hot halos and a possible decline in the number of distant cooling flows.