New Perspectives on Cooling Flows and Cluster Radio Sources

We discuss new results from the Chandra X-ray Observatory and XMM-Newton on cluster cooling flows, emphasizing early results from our Chandra programs. We find cooling rates reduced by factors of 5–10 compared to those from earlier missions. Nevertheless, substantial amounts of keV gas appear to be cooling and fueling star formation in central dominant cluster galaxies (CDGs). The structure of the keV thermal gas is remarkably complex, particularly in regions surrounding the the radio source and sites of star formation in CDGs. The radio sources are displacing the thermal gas leaving cavities filled with radio emission. The cavities are apparently supported against the local gas pressure by magnetic fields and cosmic rays. In addition, radio-faint, “ghost” cavities are seen in some clusters. They may be relics of earlier radio outbursts rising buoyantly in the intracluster medium. The radio sources may reduce the mass deposition rates by mechanical heating, and by inducing convective currents that move cool material outward. 1. The Cooling Flow Problem Roughly half of clusters of galaxies have bright cusps of X-ray emission in their central ∼ 100 kpc. The cusps are associated with so-called cooling flows: regions of dense gas with short radiative cooling times (Fabian 1994). Absent a significant source of heat, the gas will cool to low temperatures and accrete onto the cluster’s central dominant galaxy (CDG). The cooling gas will presumably accumulate there in atomic and molecular clouds and form stars. Indeed, the likelihood that a CDG has detectable levels of cold gas and star formation increases dramatically Dept. of Physics & Astronomy, Ohio University, Athens, OH U.S.A. MIT/CSR, Cambridge, MA U.S.A. Harvard-Smithsonian Center for Astrophysics, Cambridge, MA U.S.A. Engineering Physics, University of Wollongong, Wollongong, AU. Dept. of Astronomy, University of Virginia, Charlottesville, VA U.S.A.