Paula Cleggett-Haleim Headquarters, Washington, D.C. January 4, 1993 (Phone: 202/358-0883) 11:30 A.M. EST Jim Elliott Goddard Space Flight Center, Greenbelt, Md. (Phone: 301/286-6256) Release: 93-1 MYSTERIOUS CONCENTRATION OF DARK MATTER DISCOVERED Astronomers have discovered a huge concentration of mysterious "dark matter" using the international ROSAT X-ray observatory. The discovery appears to confirm previous suggestions of where most of the dark matter in the universe may be concentrated, namely in and around small groups of galaxies, according to astronomers. "The new findings add much weight to the theory that most of the mass of the universe consists of dark matter, the precise nature of which remains unknown to scientists," said John S. Mulchaey of the Space Telescope Science Institute, Baltimore, Md., and the University of Maryland, College Park. Dark matter is believed to exist although it has never been seen because it emits no radiation. Its existence has been inferred because fluctuations observed in the Big Bang -- the explosion presumed to have created the universe -- did not have sufficient gravitational pull to cause ordinary matter to coalesce immediately. It is presumed, therefore, that unknown, or dark, matter that would be attracted to the weak gravity of the fluctuation got the process started. Scientists also presume the existence of the invisible material because the speeds with which certain galaxies are moving, their rotational patterns and their shapes cannot be accounted for by the possible gravitational pull of observable matter close enough to influence them. Dark matter may, in fact, constitute up to 95 percent of the mass of the universe. Confirming its existence and volume would mean that there might be enough mass in space to "close the universe." This means that eventually the expansion of the universe, which is being slowed by the pull of gravity, would come to a halt or nearly so. - more - - 2 - The discovery announced today was made with x-ray pictures of three galaxies known as the "NGC 2300 group," located about 150 million light-years from Earth in the direction of the northern constellation Cepheus. (One light-year is the distance light travels in 1 year, approximately 5.8 trillion miles.) The images were taken with the Position Sensitive Proportional Counter instruments on ROSAT during April 25-27, 1992, according to Dr. Richard F. Mushotzky, NASA Goddard Space Flight Center, Greenbelt, Md. They show that the small group of galaxies is immersed in a huge cloud of hot gas, about 1.3 million light-years in diameter, he explained. 500 Billion Times The Sun Astronomers estimate that the cloud has a mass equal to 500 billion times that of the sun and is at a temperature of approximately 18 million degrees Fahrenheit (10 million degrees Kelvin). "A cloud like this would have dissipated into space long ago, leaving nothing for us to detect, unless it was held together by the gravity of an immense mass," Mushotzky said. "The mass required to restrain the cloud is about 25 times greater than the mass of the three galaxies that are present." This is the first time that a multimillion degree gas has been found to pervade a small group of galaxies, the Goddard astronomer explained, although such gas has been detected in larger clusters of galaxies by earlier satellite telescopes such as NASA's High Energy Astronomy Observatory-2. Results from the Hubble Space Telescope and other satellites already have shown that if the leading version of the Big Bang theory is correct, then 90 to 95 percent of the mass in the universe must be in the unknown "dark" form, astronomers explain. This means, they say, that there must be 10 to 20 times as much dark matter by mass as ordinary matter, which scientists call "baryonic matter." However, in locations observed previous to this work, the ratio of dark to ordinary matter has been, at most, a factor of two to four. Much of that work concentrated on studies of the most prominent groups of galaxies in space, the "rich clusters," according to astronomers. "Rich clusters" are huge aggregates of hundreds to thousands of galaxies. "Although they stand out the best and therefore, have been the subject of most of the research on dark matter, they are not representative of the universe, because most galaxies are in small groups like the NGC 2300 group," Mulchaey explained. "The universe is like the pre-industrial United States, in which the most conspicuous population concentrations were in a few big cities, but in which most people actually lived in small towns and rural America," he said. - more - - 3 - If small groups of galaxies all have comparable ratios of dark to ordinary matter, meaning a factor of 12 to 25 as found in the NGC 2300 group rather than a factor of four or less as found in rich clusters of galaxies, Mulchaey explained, then the mystery of where most of the dark matter in the universe is located has been solved. Closed Universe That would mean that there might be enough mass in space to "close the universe," indicating that eventually the expansion of the universe, which is being slowed by the pull of gravity, would come to a halt or nearly so, he continued. Although some scientists have suggested that the dark matter might be preferentially concentrated in small groups of galaxies, direct evidence was lacking until ROSAT observations were made, according to the astronomers. Further work is needed to confirm a discovery of this apparent magnitude, they admit. This work represents the first case in which the amount of dark matter in a small group of galaxies has been determined accurately, the science team reported. "For confirmation, we need repeated x-ray observations from space of the NGC 2300 group and other representative small groups of galaxies," Mushotzky explained. "Meanwhile, if we are right, the theorists need to start thinking about why there is much dark matter where there is little ordinary matter (i.e. in small groups of galaxies), and there is much less dark matter where there is a lot of ordinary matter (in rich clusters of galaxies)," he said. The discovery is to be announced today at a meeting of the American Astronomical Society in Phoenix, Ariz., by Mulchaey; David S. Davis, NASA Goddard Space Flight Center and the University of Maryland; Dr. Richard F. Mushotzky; and Dr. David Burstein, Arizona State University, Tempe. Mulchaey and Davis are University of Maryland graduate students who work with Dr. Mushotzky's research group at the Goddard Space Flight Center. Mulchaey currently is employed at the Space Telescope Science Institute. Dr. Burstein is a Professor of Astronomy at Arizona State. ROSAT, an acronym for Roentgen Satellite, is a joint project of Germany, the United States and the United Kingdom. It was launched on a Delta II rocket from the Cape Canaveral Air Force Station, Fla., on June 1, 1990. - end - EDITORS NOTE: A video and photograph to illustrate this story is available by calling NASA's Broadcast and Imaging Branch on 202/358-1741. Color B&W 92-HC-730 92-H-791