Like cosmic bumper cars, four galaxies are ramming into each other in one of the biggest collisions ever recorded. The quartet will ultimately merge into a single galaxy that may be several times as massive as the Milky Way.
Kenneth Rines of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and his colleagues spotted a fan-shaped plume of stars, 150,000 light-years long, spilling from a compact group of four galaxies in a cluster known as CL0958+4702. The cluster lies 5 billion light-years from Earth. Three of the galaxies weigh about the same as the Milky Way, while the fourth is about three times as massive.
Simulations suggest that the plume contains stars tossed out when one or more of the smaller galaxies had an earlier near-miss of a collision with the most massive galaxy. The galaxies, now about 55,000 light-years apart, are spiraling toward each other, Rines and his colleagues report in the Aug. 10 Astrophysical Journal Letters.
Observations from NASA’s infrared Spitzer Space Telescope and the visible-light WIYN telescope atop Kitt Peak in Arizona show that the plume contains old, red stars—exactly the type expected to make up the main stellar population in old elliptical galaxies. Moreover, the colors of the galaxies themselves indicate that they contain little hydrogen gas, the raw material for making new stars.
The stars in both the plume and the galaxies probably formed during the first 3 billion years of cosmic history in smaller galaxies that merged about 7 billion years later to form the four galaxies. The ongoing collision not only documents how a giant galaxy is made, but also enables astronomers to separate the usually intertwined epochs of star formation and galaxy assembly, Rines says.
