Dark Matter Found in a Typical Cluster of Galaxies
4 January 1993Astronomer John S. Mulchaey, of the Space Telescope Science Institute (STSCI) and fellow team members announced today the discovery of a huge concentration of mysterious "dark matter" with the ROSAT X-ray observatory satellite. ROSAT is an acronym for Roentgen Satellite, a joint project of Germany, NASA, and the U.K.)
The discovery implies that most of the dark matter in the universe is concentrated in and around small groups of galaxies, according to Mulchaey. (Our Milky Way Galaxy inhabits just such a small group.) "These groupings may represent the long sought for 'fair sample' of how dark matter is distributed in of the universe," said Mulchaey.
The new findings add support to theoretical models of the Big Bang which predict that most of the mass of the universe consists of dark matter.
The discovery will be announced today at the meeting of the American Astronomical Society in Phoenix, AZ by Mulchaey, and co-investigators David S. Davis of the NASA Goddard Space Flight Center, Greenbelt, MD and the University of Maryland, Dr. Richard F. Mushotkky of the NASA Goddard Space Flight Center, and Dr. David Burstein of Arizona State University, Tempe.
"Dark matter" means matter of an unknown type that must make up most of the mass in the universe, according to current theoretical models and indirect observations. Although astronomers had previously suggested that the dark matter might be preferentially concentrated in small groups of galaxies, direct evidence was lacking until the ROSAT observations.
The discovery was made with X-ray pictures of a small cluster of galaxies known as the NGC 2300 group. The cluster lies 150 million light years from Earth in the direction of the northern constellation Cepheus.
ROSAT's X-ray images, taken with the Position Sensitive Proportional Counter instrument during April 25-27, 1992, show that the small group of galaxies is immersed in a huge cloud of hot gas about 1.3 million light-years in diameter.
The clue to the presence of dark matter is the fact that the hot cloud should have dissipated into space long ago unless it was held together by the gravity of an immense mass.
Mulchaey explained that the cloud is maintained by a balance between the force of gravity from the dark matter which tries to compress the cloud and the pressure of the hot gas which keeps the cloud "inflated".
The team calculated the cloud's gas pressure by measuring the temperature from the ROSAT observations which turn out to be 10 million degrees Kelvin (about 18 million degrees Fahrenheit). The required mass to contain this hot gas must be about 15 to 25 times greater than the combined mass of the three galaxies alone, according to Mulchaey.
The Search For Dark Matter
Models describing the origin of helium and other light elements during the birth of the universe, or "Big Bang", show that less than 5% of the universe is made up of "normal stuff," such as neutrons and protons. This means more than 90% of the universe must be in some unknown material that does not emit any radiation that can be detected by current instrumentation.
Although this dark matter cannot be seen, its existence has been inferred from its gravitational influence on the motions of stars in galaxies and the motions of galaxies in clusters. Its presence can also be surmised from the relative amounts of light elements and isotopes produced in the Big Bang, and from the properties of high-temperature gas located in clusters of galaxies
Results from the Hubble Space Telescope and other instruments 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. This means there must be 10 to 20 times as much dark matter, by mass, as ordinary matter, according to astronomers.
However, in locations observed prior to the work done by the Mulchaey, Davis, Mushotzky, and Burstein team, the ratio of dark matter to ordinary matter has been at most a factor of 2 to 4. Much of that work concentrated on studies of the most prominent groups of galaxies in space, the "rich clusters" - huge aggregates of hundreds to thousands of galaxies.
Mulchaey's group reasoned that although rich clusters have been the subject of most of the research on dark matter, they are not representative of the universe. "In fact, most galaxies are in small groups," Mulchaey explained.
Mulchaey points out that another major difference between the NGC 2300 group and rich clusters of galaxies is that the hot gas in the smaller system is less enriched in "heavy" elements.
Light elements such as hydrogen and helium were produced in the original "Big Bang" while heavy elements such as iron are produced in stars and stellar explosions such as supernovae.
Because the remnant gas from the "Big Bang" should have little or no heavy elements, this might show that the gas in the small cluster is more "primordial" than that found in rich clusters and is a more representative sample of the universe.
A New Light on Dark Matter?
These results might solve the mystery of where most of the dark matter exists in the universe if further studies show that small groups of galaxies, which are located throughout the universe, have comparable ratios of dark to ordinary matter.
The consequences are that there is enough mass - and hence gravity - in space to "close the universe" by bringing its expansion to a halt, or nearly so.
Mulchaey emphasized that further observations of other small groups of galaxies are needed to confirm this theory. "While X-ray measurements of other small groups have not yet been done, we can speculate that based on this system, such groups may represent the place in the universe where the amount of dark matter is consistent with the models for the origin of the universe."
Mulchaey and Davis are University of Maryland graduate students who work with Dr. Mushotzky's research group at the Goddard Space Flight Center. Dr. Burstein is a Professor of Astronomy at Arizona State University.