Astronomers suspect that most galaxies have been involved in collisional encounters during their lifetimes. Galaxy-galaxy interactions stimulate the vigorous formation of stars, as gravitational effects during the encounters induce interstellar gas to condense into stars. These starbursts in turn light up the galaxies, especially at infrared wavelengths, making some systems hundreds or even thousands of times brighter than the Milky Way while the starbursts are underway. Luminous galaxies not only shed some light on how galaxies evolve and form stars, they act as lanterns that can be seen across cosmological distances, thereby helping scientists study the relatively early universe.
Galaxies tend to lie in clusters: the Milky Way and its neighbor Andromeda, for example are only the two most massive members of a cluster of about 40 galaxies that make up the "local group" of galaxies, all of which are bound together by their gravitational attraction. Galaxies that are close together and bound by gravity are very likely to have experienced collisions -- or even mergers -- during their history. Numerical simulations suggest that smaller galaxies often get absorbed into larger ones over time, but these models are still being perfected.
SAO astronomers Kenneth Rines and Alexey Vikhlinin, together with a colleague, have discovered what may be the largest major galaxy merger ever seen, one that appears to have nearly completely disrupted one of its members. The cluster itself is a known emitter of X-rays located in the not-too-distance universe -- its light has been traveling towards us for about 4 billion years. While examining infrared images of the region taken with the Infrared Array Camera (IRAC - the IRAC team is led by SAO astronomer Giovanni Fazio) on the Spitzer Space Telescope, the astronomers discovered an unusual plume of stars stretching outward over nearly a third of a million light-years. Analysis of the starlight indicates that the stars are old -- not young like the stars that commonly form when galaxies collide. The total amount of diffuse light being emitted in this process equals that of one hundred billion Suns, making this event the most massive merger ever identified.