Colossal Four-Galaxy Collision Discovered

Sara Bennington McPherson
for National Geographic News
August 7, 2007
Four galaxies are crashing into each other in one of the largest collisions ever seen, scientists say.

The galactic crash was spotted by astronomers using NASA's Spitzer Space Telescope, which detected a fan-shaped plume coming from a cluster of galaxies nearly five billion light-years away.

When fully merged, the new galaxy will be up to ten times as large as the Milky Way, astronomers said.

"Most galaxy mergers are like small pickup trucks filled with sand colliding," explained Kenneth Rines, an astronomer at the Harvard-Smithsonian Center for Astrophysics.

"This big merger is like two big rigs full of sand colliding and flinging sand everywhere. In this case, the sand represents stars."

Common Occurrence Made Unique

Galactic mergers are fairly common, Rines explained. (Read related story: "Earth Likely to Relocate in Galactic Collision" [May 16, 2007].)

Most cosmic crashes involve two galaxies of similar size or smaller galaxies coalescing into a larger one.

What makes this event unique is the sheer size and number of galaxies involved, Rines said.

"This is a very unusual case," Rines said. "It's a first to have four galaxies merging."

Three of the star systems are about the size of the Milky Way, and the fourth is about three times as large.

(Download a wallpaper photo of the Milky Way.)

Another unique aspect of the merger is the apparent lack of new stars being formed, Rines added.

Typically when galaxies converge, the intervening gas clouds compress and begin to form stars, he explained. But scientists have not detected gas clouds in the four galaxies, which means no new stars will be born from the merger.

Rines' team will publish the discovery in an upcoming issue of Astrophysical Journal Letters.

Benefit of Happenstance

Rines said the megacollision will help scientists learn more about how large galaxies are formed.

"This merger tells us that you can make a clear distinction between when a star in a galaxy forms and when the galaxy itself assembles," he said.

In this case, all of the stars had formed before the merger.

"But if you had just looked at the star age [once the new galaxy is fully formed], you would have assumed the galaxy is much older than it really is," he noted.

Rines hopes to see more of these megamergers, but he admits it was happenstance that his team spotted the new one during a survey of distant galaxy clusters.

He's also not holding out hope that he'll see the monster galaxy fully formed in his lifetime—it will take about a hundred million years for these four galaxies to finally become one.

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