SHOCKS, COLD FRONTS AND SUPERMASSIVE BLACK HOLES OUTBURSTS IN CLUSTERS OF GALAXIES

摘要

Clusters of galaxies are the most massive gravitationally collapsed systems in the Universe, with total masses as large as ~ 10~(15)M_⊙. The dominant form of mass in clusters is dark matter. Most of the "visible" matter is in the form of a hot (10~7 to 10~8K) diffuse gas, which comprises ~ 85% of the baryonic mass in clusters, with the remainder primarily being the stars in galaxies. Since the hot X-ray emitting gas traces the cluster gravitational potential, it provides an excellent means for mapping cluster structure and determining the total mass and its distribution. Einstein and ROSAT surveys of several hundred low redshift clusters detected significant substructures in ~50% of the systems, showing conclusively that most clusters were still growing through subcluster mergers. With the high angular resolution of Chandra and XMM-Newton, coupled with multiwavelength observations, cluster mergers are being studied in great detail. Cluster mergers often exhibit "cold fronts", contact discontinuities seen at the interface between the merging, less massive and thus cooler gas subcluster and the hotter cluster gas. Measurements of the gas temperature and density across the fronts allow the total velocity of the merging subcluster to be determined. Cold fronts also provide excellent sites for measuring the microscopic properties of the gas, in particular the degree to which thermal conduction must be reduced from the Spitzer rate and the suppression of transport processes. Although the infall velocities of most subclusters are found to be near the sound speed, the rare examples of supersonic mergers and their associated shock fronts allow both additional constraints on the microphysics of the gas, as well as the possibility of observing the spacial separation of the baryonic gas from the dark matter. Prior to the launch of Chandra and XMM-Newton, the clusters not undergoing mergers, particularly the set of clusters generally referred to as "cooling flow" clusters, were still believed to be relaxed, at least in their cores. But, instead of the expected uniform intracluster medium (ICM) in these clusters, high resolution X-ray images show that cluster cores are often perturbed, either by outbursts from supermassive black holes residing in the nuclei of the central dominant cluster galaxy or by the "sloshing" of the core gas due to the "flyby" of a perturbing subcluster. Jets, driven by the AGN (active galactic nuclei) outbursts, can produce cavities in the hot ICM and associated shocks that may be detected through jumps in the gas density and temperature. From measurements of cavity positions and the gas mass they displaced and from the shock strength, the outburst history of the supermassive black hole can be determined. Observations show that outbursts are common and often recurrent and that the kinetic energies of the AGN are much greater than their radiative energies. This paper reviews recent results on cluster mergers and on the effects of the outbursts from supermassive black holes on the surrounding hot atmospheres. The X-ray images, as well as the understanding of how these processes occur and effect the growth of both clusters and galaxies have been spectacular.