John Mulchaey

Division Director/ Science Deputy


Most galaxies in the universe, including our own Milky Way, exist in collections known as groups. Groups are the most common galaxy systems and are important laboratories for studying the processes associated with galaxy formation and evolution. John Mulchaey studies galaxy groups to understand the processes that affect most galaxies during their lifetimes.

As a graduate student, Mulchaey led the team that discovered that some groups are bright X-ray sources. Using space-based X-ray telescopes, he and collaborators have shown that in some groups the X-ray emission is spread over the entire group volume. The extent of the emission suggests that the X-rays originated in a very hot, low-density gas called the intragroup medium. X-ray observations of the medium have shown that the temperature is a scorching 10 million degrees. Temperatures this high should quickly disperse; but the intragroup medium does not. Astronomers believe that gravity is binding it. However, the mass required to confine the gas is much higher than the visible group mass indicating that galaxy groups are dominated by dark matter—the elusive material that does not emit light but has a strong gravitational pull.

Although Mulchaey works extensively with space-based, X-ray telescopes, the optical telescopes at Las Campanas play a central role in his research as well. X-ray images alone are not sufficient to uncover the nature of galaxy groups. Follow-up observations with large-aperture optical telescopes, such as Magellan, are necessary to determine galaxy type and redshift, or distance.

The large size of the Magellan telescopes has allowed Mulchaey to study distant galaxy groups for the first time. By studying groups at a variety of distances—and therefore times—Mulchaey is directly tracing how the group environment affects properties of individual galaxies. These observations suggest that galaxy-galaxy merging is very common in groups. For some groups, the galaxies may continue merging until they form a single massive galaxy. In the last few years, Mulchaey has uncovered several of these "fossil group" systems. Studies of them are proving to have important clues into the likely end state of most groups, including the Local Group, where our Milky Way resides.


Ph.D., 1994, University of Maryland

Galaxy groups, X-ray astronomy