Astronomers have discovered a lonely arc of debris that may be a remnant of a high-speed galactic collision. The arc of stars and gas, which seems unattached to any galaxy, is the first of its kind to be found by astronomers but conforms to theoretical models of what happens when two large galaxies collide. The work is described in a paper submitted to the Monthly Notices of the Royal Astronomical Society.

Many galaxies have long tails of gas and stars, usually stretched out by friction from slow mergers with other galaxies. But galaxies whirling around in clusters move faster than 1000 kilometers per second--much too quick for a gentle merger. However, a violent encounter could leave something akin to the faint arc, some 260,000 light-years long, spotted by Neil Trentham of the Institute of Astronomy in Cambridge and Bahram Mobasher of Imperial College in London, while searching the Coma Cluster with the University of Hawaii's 2.2-meter telescope on Mauna Kea. Says Trentham, "When two big galaxies bang together at high speeds, you should get these huge arcs of debris shooting out."

The finding is welcome news to Neil Katz, an astronomer at the University of Massachusetts, Amherst, whose simulations of high-speed galaxy encounters have predicted such accumulations of debris. Previous observations, he says, haven't penetrated far enough to pick up the faint arc.