Dead galaxies may be packed with dark matter

A study of the Coma Cluster suggests it could contain 100 times more dark matter than visible matter. 

An artist's impression of the 'quenching' process by which galaxies lose their star-forming properties
Credit: Cameron Yozin, ICRAR/UWA

Galaxies that have been sucked into a cluster 300 million lightyears from Earth could contain 100 times more dark matter than visible matter.

The Coma Cluster is found in the constellation Coma Berenices and contains thousands of galaxies bound together by gravity. A study at the International Centre for Radio Astronomy Research in Australia has looked at some of the galaxies within the cluster, revealing they contain only one per cent of the amount of stars in the Milky Way, despite being the same size.

"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,” says PhD student Cameron Yozin, who led the study using computer simulations.

Dark matter cannot be seen directly and its exact nature so far has eluded scientists. However, its existence is inferred due to its gravitational effect on visible matter. Dark matter is thought to make up about 84 per cent of all matter in the Universe.

Yozin says the galaxies he studied seem to have stopped making new stars when they fell into the cluster between seven and ten billion years ago. These are known as ‘failed’ galaxies.

 “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 says. “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.

“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. They have however avoided being ripped apart completely in this environment because they fell in with enough dark matter to protect their visible matter.”

Front image: This image from NASA shows dwarf galaxies in the Coma Cluster. In the centre of the image, two giant elliptical galaxies are seen in blue.
Credit: NASA, JPL-Caltech, SDSS, Leigh Jenkins, Ann Hornschemeier (Goddard Space Flight Center) et al.
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