Working Up the Hierarchy: Impact of AGNs on Groups and Clusters of Galaxies

Description
The observed entropy profiles (and by extension, the thermal properties) of the hot X-ray emitting gas in groups and clusters of galaxies show a much greater dispersion than expected if the gas was heated only by shocks associated with infall and mergers. This diversity is best understood as a byproduct of feedback associated with AGN activity during galaxy formation acting on the hot diffuse gas, both before and after cluster formation. Observations suggest that the level of preheating varies from one proto-cluster region to another. The entropy profiles of roughly 50% of the clusters with long central cooling times, the so-called "non-cool core" clusters, require that the gas is "preheated" to high entropy prior to cluster collapse. Those clusters that are not boosted to sufficiently high initial entropies are unstable to cooling and susceptible to massive cooling flows. We have now detected ongoing star formation in BCGs associated with low entropy systems, indicating that - contrary to today's popular view - the gas in cool core clusters IS cooling, albeit at a reduced rate. The tempered nature of the cooling flow suggests that ongoing "galaxy formation" at the centers of groups and clusters continue to influence the evolution of the intracluster medium even at present times.
Organised by Alessandro Costa