This award provides partial support for a sabbatical visit to the Max Planck Institute for Extraterrestrial Physics, Munich, West Germany, for Professor J. Patrick Henry of the Institute for Astronomy, University of Hawaii. He will participate in a major German-led astronomical survey of clusters of galaxies using satellite based x-ray equipment (ROSAT), and then conduct followup optical studies at the University of Hawaii. His primary col- laborators will be the Institute Director, Dr. Joachim Trumper, and others on the Institute staff. The ROSAT satellite was to be launched in February, 1990. The x-ray imaging survey of the entire sky is expected to provide a sample of clusters of galaxies which will be at least twice as deep as the deepest optically selected samples. The ROSAT sample will also have well established, quantifiable selection criteria and will be in a contiguous region of the sky. U.S. astronomers will have relatively little access to survey data; Professor Henry is one of very few invited to participate in the survey phase. The study of the largest structures in the universe can tell us much about its history and evolution. Galaxies are classically used to investigate structure in the universe. But surveys of galaxies are only complete to a certain scale. Clusters can give a rough idea of structure on a much larger scale. Professor Henry will be able to relay data on position clusters to observers in Hawaii. He and his colleagues will emphasize studies of the evolution of cluster x-ray and optical properties, large scale structure, and the classical magnitude redshift relation. The depth and complete- ness of the ROSAT sample will enable these studies to produce significant improvements over previous work.
