The two candidates for heating cluster atmospheres are Active Galactic Nuclei (AGNs) and thermal conduction. Heating by AGN is thought to occur through the dissipation of the internal energy of plasma bubbles inflated by the AGN at the centre of the cooling flow. Since these bubbles are less dense than the ambient gas, they are buoyant and rise through the intracluster medium (ICM) stirring and exciting sound waves in the surounding gas. This energy may be dissipated by means of a turbulent cascade, viscous processes, or aerodynamic forces. Deep in the central galaxy other processes such as supernovae and stellar winds will also have some impact on the ambient gas.
Thermal conduction may also play a significant role in transferring energy towards central regions of galaxy clusters given the temperature gradients which are observed in many clusters.
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