Image: Galaxy cluster
Credit: NASA/CXC/IfA/STScI/C. Ma
The most crowded collision of galaxy clusters has been identified by combining information from NASA's Chandra X-ray Observatory, Hubble Space Telescope and the Keck Observatory on Mauna Kea, Hawaii.
updated 4/16/2009 5:42:43 PM ET 2009-04-16T21:42:43

An interstellar pileup involving four galaxy clusters has become the most crowded cluster collision ever detected.

Astronomers spotted the galaxy clusters involved in a triple merger, the first time that such a phenomenon has been recorded. NASA's Chandra X-Ray Observatory and Hubble Space Telescope joined the Keck Observatory in Hawaii to safely observe the mess from 5.4 billion light years away.

The cosmic collision is taking place in MACSJ0717, a 13 million-light-year-long stream of galaxies, gas and dark matter known as a filament. That filamentary freeway continues to pour galaxies and other matter into a region already full of galaxies.

"In addition to this enormous pileup, MACSJ0717 is also remarkable because of its temperature," said Cheng-Jiun Ma, an astronomer at the University of Hawaii. "Since each of these collisions releases energy in the form of heat, MACS0717 has one of the highest temperatures ever seen in such a system."

Collisions between galaxies can often result in larger galaxies. Our Milky Way is slated for a run-in with a colossal cloud of gas, and eventually will merge with the neighboring Andromeda galaxy.

Researchers tracked the direction of the four clusters' motions to figure out that the filament was the main culprit behind the galaxy cluster collisions. The hottest region in MACSJ0717 is also where the filament intersects the colliding clusters, which pointed to the high number of impacts.

Computer simulations show that the most massive galaxy clusters should grow in regions where large-scale filaments of intergalactic gas, galaxies, and dark matter intersect, and material falls inward along the filaments.

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"It's exciting that the data we get from MACSJ0717 appear to beautifully match the scenario depicted in the simulations," Ma said.

Optical data from Hubble and Keck provided information about galactic motion and density along the telescopes' line of sight, while Chandra's X-ray data allowed the researchers to fully determine the 3-D motion.

Ma and his team hope to use even deeper X-ray data to measure the temperature of gas over the full 13 million-light-year extent of the filament.

The study is fully detailed in the March 10 issue of the journal Astrophysical Journal Letters.

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