The galaxy Arp 220 is actually two galaxies that have been caught in the act of merging. Astronomers think that many galaxies, including our own Milky Way, have undergone similar collisions during their histories. Although the process of galaxy collision is important and common, what happens during these encounters is not very well understood. For example, it seems likely that massive black holes (or perhaps binary black hole pairs) will form during the interactions, as the two galaxies' nuclei approach each other. Watching the process unfold helps scientists understand the evolution of the Milky Way, and, for that matter, the morphologies and distributions of galaxies throughout the universe. Arp 220, at a distance from earth of only about 250 million light years, and with a luminosity equal to that of about a trillion suns, has become the prototype for studying the merger process.
A team of seven SAO astronomers, Kazushi Sakmoto, Junzhi Wang, Martina Wiedner, Zhong Wang, Alison Peck, Qizhou Zhang, Paul Ho, and David Wilner, and a colleague, used the Submillimeter Array (SMA) to probe the two nuclei in Arp 220. The unequaled spatial resolution of the SMA at submillimeter wavelengths allowed the team to study the gas and dust in regions small enough to discover embedded structures. Indeed, they report that the dual nuclei are surrounded by a disk of gas a few thousand light-years in size, and that one of these nuclei contains warm dust apparently heated by nearby supernovae, consistent with there being active star formation present. The other nucleus, however, is different -- it is more compact, luminous, and its black hole seems to be actively accreting material (although an unusual kind of massive starburst might also explain the observations). The new results are a dramatic demonstration of the power of high spatial resolution studies at submillimeter wavelengths, and reveal the complex interactions, star forming activity, and black hole processes that can occur when galaxies collide.