Feedback in Clusters of Galaxies

Brian McNamara (University of Waterloo)

For more than 25 years, it was thought that dense gas in the cores of clusters cooled at rates of tens to hundreds of solar masses per year, and condensed in a largely unseen form. This perception changed dramatically when X-ray observations of galaxy clusters by the Chandra and XMM-Newton observatories failed to find gas cooling below X-ray temperatures at the expected rates. At about the same time, images from the Chandra observatory revealed giant cavities and shock fronts that provide a direct and reliable means of measuring the energy injected into hot atmospheres by active galactic nuclei (AGN). Average AGN powers are near those required to suppress cooling in isolated giant elliptical galaxies, and in larger systems up to the richest galaxy clusters. This coincidence suggests that heating and cooling are coupled by feedback, which suppresses star formation and the growth of luminous galaxies. I will discuss the broader implication of these results for several fundamental astrophysical problems including the growth of bulges and supermassive black holes, the excess entropy in hot halos, and baryon overcooling in the universe.