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The Sciences

Astronomers Find a Galaxy Stuffed With Dark Matter


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Just beyond the Milky Way, astronomers have found an extremely dim dwarf galaxy that appears to have just a few hundred stars, but is surprisingly massive. Researchers say the galaxy, called Segue 1, must be packed with mysterious dark matter in order to give it such bulk. Dark matter has never been directly detected, and its presence can only be deduced:

Although dark matter doesn't emit or absorb light, scientists can measure its gravitational effect on ordinary matter and believe it makes up about 85 percent of the total mass in the universe. Dark matter is thought to play a crucial role in galaxy formation, perhaps by contributing to the clumps that stimulate star formation in a budding galaxy and by contributing to the overall matter of a galaxy that allows it to lure other matter and galaxies inward in a growth-by-merger process [].

The discovery of Segue 1, which will be described in a forthcoming issue of Astrophysical Journal, gives astronomers a new venue in which to study dark matter.

The object is one of a larger group of recently discovered galaxies where dark matter seems to outweigh ordinary matter by a factor of 100 or 1000. In the Milky Way, dark matter is thought to outweigh luminous matter by a factor of 10. "These galaxies are basically invisible," says [study co-author] Beth Willman.... "We didn't know these kinds of galaxies existed until a couple of years ago" [New Scientist].

The game changed with broad surveys like the Sloan Digital Sky Survey, which scanned the skies for areas where stars appeared to be slightly more dense than usual. Researchers are now wondering if these dim, dwarf galaxies are, in fact, the norm:

"For every bright galaxy we can detect in the sky, there are likely hundreds of ultra-faint dwarf galaxies surrounding them that we are unable to detect with current technology," [said co-author] James Bullock.... "It seems that these nearly-invisible galaxies are likely the most common galaxies in the universe" [Cosmos].

Image: M. Geha

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