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What do you get if you take four galaxies and set them on a collision course? The biggest cosmic pile-up we earthlings have ever seen. Come back in a few million years and they will have formed a giant galaxy, roughly ten times the size of our puny Milky Way

Four galaxies collide. Image Credit: NASA/JPL-Caltech/K. Rines (Harvard-Smithsonian CfA)

The star-wreck was spotted by astronomers using NASA's Spitzer space telescope and the WIYN telescope. (WIYN, a ground based imaging scope capable of capturing extremely high resolution images, is jointly run by the universities of Wisconsin, Indiana, Yale and the National Optical Astronomy Observatory.)

Collisions between galaxies are not unusual, but this is a particularly spectacular event. It is more common to see a smaller galaxy, or even several smaller galaxies, being swallowed by a larger one. Astronomers have also seen mergers between pairs of similarly sized galaxies, but they have never seen four such juggernauts barrelling along towards each other like this.

Kenneth Rines of the Harvard-Smithsonian Center for Astrophysics describes the crash: "Most of the galaxy mergers we already knew about are like compact cars crashing together. What we have here is like four sand trucks smashing together, flinging sand everywhere."

The slow-motion merger was spotted during a routine survey of a distant galactic cluster, roughly 5 billion light years away. The first indication that something was up came when the astronomers spotted a fan-shaped plume coming out of four close-packed galaxies.

Fine detail in the WIYN pictures also suggests the galaxies involved in the merger are elliptical rather than spiral galaxies. And closer analysis of the plume revealed that it is composed of billions of older stars.

"The WIYN images show that the four galaxies have well-defined cores that have held together during the merger, much like egg yolks stay together longer than egg whites if you "merge" them in a mixing bowl," said Rines

"The Spitzer data show that these major mergers are gas-poor, unlike most mergers we know about," he continued. This means new stars are unlikely to form in the giant that emerges from this collision.

The work is published in the Astrophysical Journal Letters 665:L9-L13, (August 10, 2007). ®

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