Galaxy loaded with dark matter, Mauna Kea telescope suggests

A faint galaxy that is orbiting the Milky Way has proved to be a plentiful lode of mysterious dark matter, observations from Mauna Kea confirm.

Astronomers using the Keck II telescope say that the group of 1,000 stars known as Segue 1 make up the dimmest known galaxy.

Like an iceberg with most of its body hidden, the dwarf galaxy appears to have 3,400 times more mass than its visible stars would account for, researchers reported recently in the Astrophysical Journal.

The initial announcement of Segue 1’s "heart of darkness" was made in September 2008 by Yale University astronomer Marla Geha and Joshua Simon of the Carnegie Institution.

But other scientists didn’t buy it, suggesting instead that the movements of Segue 1’s stars could be explained in other ways.

Working with the Keck’s Deep Extragalactic Imaging Multi-Object Spectrograph, Geha, Simon and collaborators now have shown that Segue 1’s stars have wildly varying speeds, from 120 miles per second to nearly 140 miles per second relative to the Milky Way.

"That tells you Segue 1 must have much more mass to accelerate the stars to those velocities," Geha said in a news statement Friday.

If Segue 1 had very little dark matter, the stars would all move at about the same speed, the researchers say.

The astronomers estimate that the mass required to cause the different star velocities seen in Segue 1 is equivalent to 600,000 of our suns. But each of the 1,000 stars in Segue 1 is about the same size as the sun.

That leaves dark matter, which some scientists say comprises more than 80 percent of the matter in the universe. Regular matter makes up less than 20 percent.

Now astronomers are watching for gamma rays from Segue 1 that could signal dark-matter collisions.

So far, nothing. But that could just mean that the instrument in question, the space-based Fermi Gamma Ray Telescope, isn’t powerful enough.

"A detection would be spectacular," Simon said. "People have been trying to learn about dark matter for 35 years and not made much progress. Even a faint glow of the predicted gamma rays would be a powerful confirmation of theoretical predictions about the nature of dark matter."