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Joel N. Bregman (University of Michigan)
Observations obtained with ROSAT made major contributions to our understanding of spiral galaxies, elliptical galaxies, galaxy clusters, and large-scale structure. For spiral galaxies, observations of starburst galaxies revealed the outflow phenomenon in detail, while in normal spirals, discoveries include the diffuse emission from the galaxy disks as well as from the galaxy halos. Studies of elliptical galaxies helped elucidate the nature of the X-ray emission, leading to a deeper understanding of the stellar and gaseous contributions. There have been exciting studies of the galactic mass distribution and of the interaction of these galaxies with their environment.
The studies of galaxy clusters have been important for understanding their structure and evolution as well as for insights into large-scale structure. For example, ROSAT observations were central to the studies of cluster merging rates and relaxation, which can provide constraints on cosmological models. ROSAT observations have served to define the evolution of clusters to cosmologically interesting distances, and has led to the discovery of some of the most distant clusters, with z > 2! Other studies have shown that the majority of clusters contain cooling flows, with particularly interesting properties in the inner regions. Also, ROSAT made possible the first serious studies of the gas in galaxy groups, which is the most common environment of galaxies.
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