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Isolated for billions of years, a galaxy with more dark matter packed into its core than expected has been identified by astronomers using data from NASA's Chandra X-ray Observatory.

The galaxy, known as Markarian 1216 (abbreviated as Mrk 1216), contains stars that are within 10% the age of the universe—that is, almost as old as the universe itself. Scientists have found that it has gone through a different evolution than typical galaxies, both in terms of its stars and the invisible dark that, through gravity, holds the galaxy together. Dark matter accounts for about 85% of the matter in the universe, although it has only been detected indirectly.

Mrk 1216 belongs to a family of elliptically shaped galaxies that are more densely packed with stars in their centers than most other galaxies. Astronomers think they have descended from reddish, compact galaxies called "red nuggets" that formed about a billion years after the big bang, but then stalled in their growth about 10 billion years ago.

If this explanation is correct, then the dark matter in Mark 1216 and its galactic cousins should also be tightly packed. To test this idea for the first time, a pair of astronomers studied the X-ray brightness and temperature of hot gas at different distances from Mrk 1216's center, so they could "weigh" how much dark matter exists in the middle of the galaxy.

"When we compared the Chandra data to our computer models, we found a much stronger concentration of dark matter was required than we find in other galaxies of similar total mass," said David Buote of the University of California at Irvine. "This tells us the history of Mrk 1216 is very different from the typical galaxy. Essentially all of its stars and dark matter was assembled long ago with little added in the past 10 billion years."

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Category: Science