Galaxies are often found in groups or clusters. The Milky Way, for example, is a member of the "Local Group" of about three dozen galaxies, which includes the Andromeda Galaxy located about 2 million light-years away. Very large clusters can contain thousands of galaxies, all bound together by gravity. Sometimes these galaxies collide with one another, and, in even more cataclysmic events, clusters can even collide and merge with other clusters.
Compact galaxy groups contain fewer galaxies than the large clusters, and are an important class of objects because they may serve as cosmic building blocks in the large-scale structure of the universe. After galaxies themselves form in the early universe, such modest-sized groups of galaxies may be the next systems to evolve. These groups of galaxies may then combine with each other to form the bigger galaxy clusters.
The regions between galaxies in these clusters is filled with very hot, dilute gas. In the past decade images of this gas in X-rays, obtained with the Chandra X-ray Observatory, have been critical in furthering our understanding of the massive black holes that reside at the centers of many galaxies. The images often reveal jets of charged particles being ejected from the black holes into the hot gas, and these jets produce large cavities of gas seen in the X-ray emission -- providing direct evidence of the strong coupling between the black hole nuclei and the hot gas.
There has been an outstanding puzzle, however. Estimates of the total power required to produce the observed cavities in many such systems imply jet values about 100 times larger than observed. CfA astronomers Paul Nulsen and Brian McNamara and their colleagues completed a deep Chandra X-ray Observatory study of one compact galaxy group known as HCG62 containing about 60 galaxies. They were able to reexamine the energetics of this group, and to observe directly (and for the first time) X-ray emission from one of the supermassive black holes. They report that indeed the rapidly moving electrons in the jet cannot provide the support required to maintain the giant cavities observed. Instead, the scientists conclude that in this case -- and presumably other compact groups as well -- a significant amount of gas must be heated in other ways, or else perhaps there needs to be powerful cosmic ray activity. The new observations find evidence of recent merger activity in the group, however, perhaps contributing to precisely this heating of the gas, and helping to address this longstanding mystery.