Dead galaxies may be packed with dark matter

Melbourne, July 20: Galaxies in a cluster roughly 300 million light years from Earth could contain 100 times more dark matter than visible matter, a new study has found.

Researchers used powerful computer simulations to study galaxies that have fallen into the Coma Cluster, one of the largest structures in the universe in which thousands of galaxies are bound together by gravity.

"The galaxies could have fallen into the cluster as early as seven billion years ago, which, if our current theories of galaxies evolution are correct, suggests they must have lots of dark matter protecting the visible matter from being ripped apart by the cluster," researchers said.

Dark matter cannot be seen directly but the mysterious substance is thought to make up about 84 per cent of the matter in the universe.

International Centre for Radio Astronomy Research (ICRAR) PhD student Cameron Yozin, who led the study, said that the study demonstrates for the first time that some galaxies that have fallen into the cluster could plausibly have as much as 100 times more dark matter than visible matter.

Yozin, who is based at The University of Western Australia, said that the galaxies he studied in the Coma Cluster are about the same size as our own Milky Way but contain only one per cent of the stars.

He said that the galaxies appear to have stopped making new stars when they first fell into the cluster between seven and ten billion years ago and have been dead ever since, leading astrophysicists to label them "failed" galaxies.

This end to star formation is known as "quenching". "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 said.

"Falling into a cluster is one way in which this can happen. The immense gravitational force of the cluster pulls in the galaxy, but its gas is pushed out and essentially stolen by hot gas in the cluster itself," he said.

"For the first time, my simulations have demonstrated that these galaxies could have been quenched by the cluster as early as seven billion years ago," Yozin said.

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

PTI