Heating and Cooling in Clusters and Groups

TL;DR

This paper reviews the balance of heating and cooling processes in galaxy clusters and groups, emphasizing the role of black hole energy output and the tight regulation of gas temperature in cluster cores.

Contribution

It provides a comprehensive discussion of observed heating and cooling mechanisms in galaxy clusters, highlighting the near-perfect balance and proposing non-radiative cooling as a potential factor.

Findings

Heating and cooling are balanced within 20% in cluster cores.

Non-radiative cooling by mixing may explain the lack of observed cooling.

The balance is surprisingly tight despite complex processes.

Abstract

The gas in the cores of many clusters and groups of galaxies has a short radiative cooling time. Energy from the central black hole is observed to flow into this gas by means of jets, bubbles and sound waves. Cooling is thus offset by heating. We discuss the mechanisms involved and observed in the X-ray brightest clusters and explore the closeness of the heating/cooling balance. It is surprisingly tight on the cooling side when soft X-ray spectra are examined. Non-radiative cooling by mixing is suggested as a means to relax the apparent strong lack of cooling. Nevertheless the heating and cooling must balance on average to better than 20 per cent.