Clusters of Galaxies
| Professors: Cui, Lee, Peterson Clusters of galaxies are the largest and most massive, gravitational bound structures in the Universe. Clusters are complex, multi-component systems with hundreds of galaxies, a hot intracluster medium, and dark matter evolving in tightly coupled ways. Only a few percent of the mass in clusters lies in the optical galaxies. Groups and clusters are filled with a very X-ray luminous, hot (107 to 108 degree K) interstellar medium. In rich clusters, most of the luminous baryonic mass (about 20% of the total cluster mass) is in this hot intracluster medium. The properties of clusters are functions both of the processes operating within them and of the underlying cosmology. Several features make clusters ideal for testing cosmological models. First, clusters are very luminous and are observed both optically and in X-rays to significant distances (z~1). Second, since clusters are massive and relatively rare objects, they form only from fairly high peaks in the underlying density field, in the standard scenario. Third, since clusters have dynamical timescales which are a significant fraction of the age of the Universe, we can watch them evolve over even modest redshifts. Also since dynamical timescales for clusters are long, the imprint of the initial conditions has not yet been completely erased. Studies of clusters have placed tight constraints on Omega_matter and sigma_8 and are now being used to measure w, the equation of state for dark energy. For cosmological studies, the cluster mass function is perhaps the most important since it is directly traceable to the underlying cosmology and, unlike some other cluster properties, is not susceptible to modification by non-gravitational processes. For individual clusters, X-ray observations show the effects of major mergers, when a large subcluster collides with a massive cluster These observations can determine the merger velocity, the distribution of baryonic matter which can be compared with that of dark matter determined through lensing observations, and the self-interaction cross section for dark matter. Chandra observations have shown the impact on the hot gas of outbursts from supermassive black holes at the nuclei of massive galaxies at the centers of clusters, groups and isolated ellipticals. M87 in the Virgo cluster and the Perseus cluster are two examples. These outbursts are likely powered by the infall of cooling gas onto the black hole. Through weak shocks and expanding bubbles filled with radio emitting plasma, most of the cooling gas in clusters is reheated by "feedback" from the supermassive black hole. |
 Galaxy Cluster LCDCS-0829 acting like a giant magnifying glass. This unusual effect is called gravitational lensing. |