<!--intro-->Astronomers behind the Massive Cluster Survey (MACS) have uncovered 101 giant galaxy clusters, many of them so distant and thus forming so early in the history of time that they challenge our current understanding of how quickly the Universe evolved into its current hierarchical structure of stars, galaxies and clusters. <!--/intro--> Drs. Harald Ebeling and J. Patrick Henry of the University of Hawaii's Institute for Astronomy and Dr. Alastair Edge of the University of Durham, U.K., present their findings today at the meeting of the High Energy Astrophysics Division (HEAD) of the American Astronomical Society (AAS) in Honolulu, Hawaii. Their work is funded by a five-year NASA grant. <a HREF="http://www.ifa.hawaii.edu/%7Eebeling/press/macs/macs1-512.jpg"><img src="http://www.ifa.hawaii.edu/%7Eebeling/press/macs/macs1-512.jpg" width=200 height=200 border=0 align=right></a>Galaxy clusters are the largest gravitationally bound structures in the Universe, typically containing a few hundred to thousands of galaxies, each of which in turn contains many billions of stars. "MACS has been mind-bogglingly successful at finding the elusive massive, distant galaxy clusters," said Dr. Ebeling. "The high discovery rate of MACS means that there are many more of these systems out there than was previously thought, and this has profound cosmological implications." Now more than three quarters complete, MACS has identified 101 distant clusters. Previous cluster surveys detected only few such systems, leading scientists to conclude that massive clusters were not common in the distant past and must have formed relatively late in the history of the Universe, perhaps as "recent" as three billion years ago. The detection of many very distant, massive galaxy clusters -- with redshift greater than 0.3, or more than five billion light years away from us -- indicates that these giant systems began to form much earlier, when the Universe was only half its current age. This information, Dr. Edge said, can be translated directly into a measurement of omega, the density of matter in the Universe, arguably the most important and sought-after number in observational cosmology today. "MACS will allow us to measure omega with very good accuracy," said Dr. Edge. "The analysis is not yet complete, but it is already clear that our observations are in conflict with a high value of omega." The value of omega helps scientists to constrain the curvature of the universe and the value of the cosmological constant, which determines whether the universe will continue to expand forever or will collapse upon itself. Hundreds of nearby clusters have been known for a long time, from poor groups of galaxies like the one containing our own galaxy, the Milky Way, to very rich systems such as the massive cluster in the constellation of Coma less than 500 million light years away. The more distant clusters, however, hold the key to understanding how clusters form and evolve. Finding these systems had been a major observational challenge until the advent of superior X-ray observatories. MACS is being compiled from archival All-Sky Survey data from ROSAT, a German X-ray satellite with U.S. and U.K. collaboration that stopped collecting data in December 1998. X-ray observations are a very efficient means of finding and studying clusters or, more accurately, the enormous quantities of hot gas they contain. This gas, hundreds of millions of degrees Celsius hot, is invisible to optical telescopes but easily detected with X-ray cameras like the ones flown on ROSAT. MACS takes advantage of the all-sky coverage of the ROSAT survey to find the most extreme clusters which are so rare that searches over small patches of sky could never uncover them in significant numbers. While the sheer number of clusters discovered by MACS is already an important result, there is much more to be learned from these systems. The study of clusters is also fascinating because they contain great quantities of dark matter, which contributes far more to the overall cluster mass than all galaxies and gas taken together, said Dr. Ebeling. The close coexistence and interaction in clusters of the three major components of the Universe -- galaxies (i.e. stars), gas, and dark matter -- make clusters wonderful astrophysical laboratories. As the MACS team works toward the completion of the survey, a number of observatories -- the orbiting Chandra X-ray Observatory, the Subaru 8m and Keck 10m optical telescopes on Mauna Kea, and the BIMA array of nine radio telescopes in Northern California -- have begun to target the newly discovered galaxy clusters for detailed follow-up studies. With the emphasis of MACS slowly shifting from the discovery phase to an in-depth analysis of MACS cluster properties, Dr. Ebeling and his collaborators are sure that their project will provide lively fodder for years to come in the ongoing debate about the distribution and ultimate fate of matter in the Universe. The <a href="http://www.ifa.hawaii.edu/">Institute for Astronomy</a> at the University of Hawaii conducts research into galaxies, cosmology, stars, planets, and the Sun. Its faculty and staff are also involved in astronomy education, and in the development and management of the observatories on Haleakala and Mauna Kea.