Most galaxies in the universe, including our own Milky Way, exist in small collections known as groups. John Mulchaey has been interested in galaxy groups since he was a graduate student. In 1992, he led the team that discovered that some of these collections are bright x-ray sources. This discovery has provided new insights into the fundamental nature of these objects.

Using the space-based x-ray telescope ROSAT, Mulchaey and collaborators found that x-ray emission is spread over the entire volume of a group. The extent of the emission suggests that the x-rays originated in a very hot gas; observations have shown that the temperature is about 10 million degrees. Since many groups emit x-rays, the scientists believe that the gas is long-lived. However, the extreme temperature suggests that it should have already escaped the gravitational attraction from the combined mass of the galaxies and gas. The fact that the gas is still present led Mulchaey and colleagues to conclude that galaxy groups are dominated by dark matter--the elusive material that does not emit light but has a strong gravitational pull. Uncovering the nature of this dark matter remains one of the most important goals in the field of astrophysics.

Mulchaey is now using two recently launched x-ray telescopes, Chandra and XMM-Newton, in his research. Chandra is the "x-ray Hubble Space Telescope," producing images with a resolution almost 100 times better than previous instruments. Chandra allows Mulchaey to study the x-ray emission in unprecedented detail. XMM-Newton, on the other hand, is more sensitive to fainter x-ray sources. Mulchaey is using this telescope to study groups at much higher redshifts, where virtually nothing is currently known. By looking farther back in time, the XMM-Newton observations will provide insight into how groups of galaxies evolved.