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Completely dark galaxies
ROYAL ASTRONOMICAL SOCIETY NEWS RELEASE
Posted: January 5, 2001

The universe could be harbouring numerous galaxies that have no stars at all and are made entirely of dark matter. Astronomers may ultimately discover that completely dark galaxies outnumber the familiar kind populated by shining stars and gas, perhaps by as many as 100 to 1.

UGC 10214
An image of UGC 10214. Photo: RAS
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This intriguing prediction is made by Drs Neil Trentham, Ole Moller and Enrico Ramirez-Ruiz of the University of Cambridge in a paper to be published in the Monthly Notices of the Royal Astronomical Society.

There is already a considerable amount of evidence that bright galaxies contain large amounts of dark matter, often ten times more than the mass of all their stars put together. There must be extra mass that we do not see to account for the observed movements of the stars under the influence of the gravity of the whole galaxy. In some galaxies we see so few stars they are incapable of holding themselves together as a galaxy. They would have long since scattered through space without the gravity of unseen matter to keep them together.

"Observationally, a picture is emerging that there is a lot of dark matter in the universe and that most galaxies possess a great deal of it," says Neil Trentham. "On the theory side, the cold dark matter theory predicts that there are many low-mass galaxies for every massive one, but we don't see many of them around. That could simply be because very few stars - perhaps none at all - have formed in them. So the question is, 'How do we look for these completely dark galaxies?'"

It's a difficult challenge, and the best technique will depend on the nature of the dark matter, which is still unknown. Trentham and colleagues have some suggestions. If the dark matter is composed entirely of fundamental particles, dark galaxies may act as gravitational lenses, distorting the appearance of distant galaxies that happen to lie behind them. If the dark matter includes some brown dwarfs their infrared radiation may be detectable. The same will be true if the galaxies contain any dead stars, such as white dwarfs or black holes. If they are nearby, it might be possible to detect these stellar remnants acting as gravitational lenses on the light of individual stars in other galaxies beyond them. Several lensing events in a small area of sky would suggest the presence of a dark galaxy.

The researchers have identified one place where a dark galaxy may exist, using yet another phenomenon that hints at the presence of an invisible object. They noticed that a galaxy called UGC 10214 has a stream of material flowing out of it, as if it is interacting with another galaxy. But in this case, the stream of material is apparently flowing towards nothing.

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