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Dead Galaxies In Distant Cluster May Be Packed With Dark Matter

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Dead galaxies inside a huge galactic cluster, one of the largest structures in the universe, may contain up to 100 times as much dark matter as visible matter, Australian researchers say.

Scientists at the International Center for Radio Astronomy Research in Western Australia have used computer simulations applied to observational data to determine the amount of dark matter in galaxies fallen into the Coma cluster.

In the cluster about 300 million light years from Earth gravity has bound thousands of galaxies together.

"[The simulations] found the galaxies could have fallen into the cluster as early as 7 billion years ago, which, if our current theories of galaxies are correct, suggests they must have lots of dark matter protecting the visible matter from being ripped apart by the cluster," a statement from the center said.

Mysterious dark matter, which cannot be observed directly and can only be detected by its gravitational effect on the visible universe, is thought to constitute as much as 84 percent of all the matter existing in the entire universe.

The researchers' simulations suggest some of the galaxies within the Coma cluster could possibly be made up of as much as a hundred times the amount of dark matter than the amount of matter visible in direct observations, says center doctoral student Cameron Yozin.

The galaxies in question are similar in size to our own Milky Way galaxy but have only 1 percent as many stars, he says.

They likely stopped producing stars - a phenomenon known as "quenching" - when they fell into the giant cluster and have existed as what astrophysicists term "failed" galaxies ever since, he says.

"Galaxies originally form when large clouds of hydrogen gas collapse and are converted to stars - if you remove that gas, the galaxy cannot grow further," Yozin explains.

One of the ways this can occur is when a galaxy is pulled into a cluster by the cluster's immense gravitational force, the researchers say; the galaxy gets pulled in but its hydrogen gas is drawn away, in essence stolen by greater masses of hot gas within the cluster itself.

Despite the quenching and the gravitational forces of the cluster acting on the galaxies falling into it, they've remained intact - something of a puzzle for which the presence of dark matter could be one solution, Yozin says.

That could explain why they're still intact after 7 to 10 billion years, he says.

"They have ... avoided being ripped apart completely in this environment because they fell in with enough dark matter to protect their visible matter," Yozin argues.

The researchers' study has been published in the Monthly Notices of the Royal Astronomical Society.

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