Two giant clusters of galaxies have been spotted slamming into one another in a distant collision that might help researchers understand how huge groups of galaxies evolve.

Galaxy clusters are thought to be the largest structures in the universe. Collisions between them may produce more energy than any events since the Big Bang, say researchers involved in the work.

Melanie Johnston-Hollitt, a student at the University of Adelaide in Australia, found the wreckage of a collision between two of these immense structures by studying radio-wave observations. The research, which was first discussed last year, was presented Tuesday, Feb. 6 at a workshop on galaxy clusters in Sydney.

"Space is big. The chance of things running into each other is small," said Ron Ekers, director of the Australia Telescope National Facility, where the work was done. "Until now there has been only weak evidence that clusters might collide."

Clusters are big groups of galaxies, huddled and held together by gravity. Johnston-Hollitt studied a cluster of about 500 galaxies called Abell 3667, which lies 700 million light-years away. Abell 3667 appears to have run into a slightly smaller galaxy cluster.

The key evidence is a pair of arcs of radio emission that straddle the cluster, 12 million light-years apart. As viewed from Earth, the distance between them is about twice the diameter of the Moon. But the readings are very faint.

The collision and its aftermath are like "the Titanic hitting an iceberg," said Johnston-Hollitt. "Afterwards you see only ripples and bits of wreckage, but that's enough to show that there's been a collision."

Theorists have predicted that galaxy clusters would fall together at thousands of miles (kilometers) a second, smacking into each other and producing huge shock waves in the thin hot gas that fills the space between the galaxies. In 1999, researcher Kurt Roettiger, then of University of Missouri-Columbia, and colleagues suggested that the shock waves would produce large arcs of radio-emitting particles on the outskirts of the cluster, like those of Abell 3667.

The collisions might explain the origin of ultrahigh-energy cosmic rays, which remain a mystery.

"No process in our galaxy can make them," said the University of Adelaide's Roger Clay, a cosmic ray researcher and one of Johnston-Hollitt's thesis supervisors. "Perhaps shock waves in merging clusters power them up."

The observations were made using the Australia Telescope, operated by the Commonwealth Scientific and Industrial Research Organization's (CSIRO).

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