Colliding galaxies shed light on dark matter

WASHINGTON Wed Aug 27, 2008 7:25pm EDT

MACSJ0025.4-1222 in an undated Hubble and Chandra composite image of the galaxy cluster. Astronomers have captured images of a powerful collision of galaxy clusters and say it may shed light on the behavior of dark matter. REUTERS/NASA/Space Telescope Science Institute/Handout

MACSJ0025.4-1222 in an undated Hubble and Chandra composite image of the galaxy cluster. Astronomers have captured images of a powerful collision of galaxy clusters and say it may shed light on the behavior of dark matter.

Credit: Reuters/NASA/Space Telescope Science Institute/Handout

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WASHINGTON (Reuters) - Astronomers have captured images of a powerful collision of galaxy clusters and say it may shed light on the behavior of dark matter.

They used NASA's Chandra X-ray Observatory and Hubble Space Telescope to study the cluster, known a MACSJ0025.4-1222.

They can see a clear separation between dark and ordinary matter, answering a crucial question about whether dark matter interacts with itself other than via gravitational forces, the researchers said on Wednesday.

"Dark matter makes up five times more matter in the universe than ordinary matter," said Marusa Bradac of the University of California Santa Barbara, who led the work.

"This study confirms that we are dealing with a very different kind of matter, unlike anything that we are made of. And we're able to study it in a very powerful collision of two clusters of galaxies," Bradac said in a statement.

Using optical images from Hubble, the team was able to infer the distribution of the total mass of both dark and ordinary matter in the cluster using a technique known as gravitational lensing.

This method uses the distortion that mass causes as light passes by another object between the viewer and whatever is being observed. Dark matter cannot be directly seen but it has mass and thus gravitational pull.

The Chandra X-ray images showed more clearly where ordinary matter, in the form of hot gas, was.

As the two clusters collided and merged at speeds of millions of miles (km) per hour, the hot gas in each cluster collided and slowed down, but the dark matter did not.

The researchers are looking into the past with their observations. MACSJ0025 is 5.7 billion light years away, a light year being the distance light travels in a year, or 6 trillion miles.

(Reporting by Maggie Fox; Editing by Julie Steenhuysen and Cynthia Osterman)

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