Clusters and Cosmology

Most of the matter in the Universe is "dark". Although emission directly from this material has not been detected at any wavelength, the presence of this matter is inferred from observations at many wavelengths from the radio, through the optical, to the X-ray. These observations have shown that galaxies are surrounded by dark matter halos and that clusters of galaxies are filled with dark matter.

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 (see Bullet cluster image). 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 (see image) 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.

Much of the cluster science is performed through collaborations with scientists at other institutions. The CfA scientists actively working on X-ray observations of clusters, groups and galaxies and their current projects include:

• Deep Chandra observations of M87, MSO735, Hercules A, NGC5813, and other clusters to study in detail the energy feedback from the supermassive black hole to the surrounding gas. -- Bill Forman, Paul Nulsen, Larry David, Ralph Kraft, Scott Randall, and Christine Jones
• Chandra observations of large samples of clusters to measure the evolution in the mass function, scaling relations, and constrain cosmological parameters. Of particular note is the 400 square degree X-ray cluster survey (see http://hea-www.harvard.edu/400d) -- Alexey Vikhlinin
• Deep Chandra observations of merging clusters. By measuring the density and temperature jumps in the merging subclusters, the velocity of the merger can be determined. Along with lensing observations to map the total mass, Chandra observations of the "Bullet" cluster show that the dark matter and visible mass (the X-ray gas) are offset, requiring the existence of dark matter. -- Alexey Vikhlinin, Scott Randall
• Chandra and GMRT radio observations of X-ray cavities in clusters, groups and galaxies produced by outbursts from the supermassive black hole -- Jan Vrtilek, Bill Forman.
• Interactions of galaxies with the hot intracluster medium. X-ray observations often show long ram pressure stripped tails from both early-type and disk galaxies in groups and clusters. These observations often allow the velocity and direction of motion of the galaxy to be measured. With Spitzer observations, star formation associated with the galaxy's interaction with the hot ICM can be studied. -- Marie Machacek, Scott Randall
• Chandra's superb spatial resolution has allowed essentially all of the soft X-ray background to be resolved into sources. Combining Spitzer and Chandra observations has provided a powerful tool for studying AGN, particularly obscured sources.