Sept. 18, 2007 — The 70-year effort to unravel the mysteries of dark matter just got a big boost from some very puny galaxies.

In the past few years a score of dwarf galaxies have been discovered hanging about the fringes of the Milky Way. Now new measurements of the few stars in these dwarfs reveal them to be dark matter distilleries, with upwards of 1,000 times more dark matter than normal matter.

As a result, the dwarfs seem like a likely place to learn more about the elusive stuff.

Dark matter has long stumped scientists since it is something that has gravity but appears to not interact in any other way with the normal matter that makes up people, the planets and the stars.

The reason astronomers suspect dark matter exists is that measurements of the quantities of visible matter in galaxies, and stars within galaxies, persistently come up short on matter to explain how they were moving.

It was as if they had invisible dance partners. Exactly what the dance partner — this dark matter — is, however, remains a nagging enigma.

The current most favored theory is that it is some sort of particle that interacts only extremely rarely with normal matter in any other way than tugging its gravity.

Physicists hope to settle the question in the next few years by possibly creating dark matter particles in the world’s largest particle accelerator, the Large Hadron Collider, set to begin working next year at CERN, a particle physics laboratory in Geneva.

In the meantime, space is still the best place to detect the effects of dark matter and narrow down the constraints on what sort of particles dark matter might be. The newfound dwarf companions to the Milky Way are now presenting a rare opportunity to learn details about how dark matter behaves on a relatively small scale very close to home.

"There are many ways to look at larger-scale distribution of dark matter," said dwarf galaxy investigator Beth Willman, a fellow at the Harvard-Smithsonian Center for Astrophysics.

Already there are lots of models looking at the overall distribution of dark matter in the universe, which help to make sense of the structure of the universe in the largest scale.

"But as you go to smaller and smaller scales," she said, "the predictions of those models can be quite different."

o use a road atlas analogy, dark matter models can recreate a pretty accurate interstate highway map of the universe, but they aren't so good when you need to get down to the state or province map level. The newly located dwarf galaxies are dark matter clumps on the scale of a city map — right in our own galactic neighborhood, which is why astronomers are so excited to find them.

The dwarf galaxies were discovered using the Sloan Digital Sky Survey, which looked at about a quarter of the sky. It automatically identified unusual clusters of stars that warranted further study by astronomers.

Astronomers Marla Geha and Josh Simon have done just that, using the 10-meter Keck II telescope at the W.M. Keck Observatory in Hawaii. They gathered light from 814 stars in eight dwarf galaxies and split the light into spectra — their rainbows of colors, or wavelengths. Lines found in these spectra show how fast the stars are moving.

To their surprise the stars were stellar snails, poking along at about 8,900 to 15,000 miles per hour (4 to 7 km per second), the slowest stars ever measured in any galaxy. Our own sun, for a more typical example, spins around the galaxy at about 490,000 miles per hour (220 km per second).

Such slow speeds with so little visible matter, in the form of stars and gas, mean the stars are slogging through a whole lot of dark matter. The duo's results will be published in the Nov. 10 issue of Astrophysical Journal.

Dwarf Census

The finding leaves researchers with two key questions, including, are there more dwarfs swarming around the Milky Way? And how did these dwarfs come to be?

"It's kind of a 'looking for a needle in a haystack' kind of thing," said Taft Armandroff, director of the W. M. Keck Observatory. First you need a high quality survey that can both look at a lot of sky and automatically recognize very subtle collections of stars. Then you need a powerful telescope with a very good spectroscope to analyze the light from the stars.

More surveys are being planned, but they could fail to find many more dwarfs for another, rather ironic reason: because these dwarfs have so much more dark matter than stars, many could have just a handful of stars. That would make them almost impossible to identify, said Geha.

Dwarf-Making

As for how the dwarfs were born, there are two theories, says dark matter theorist James Bullock of the University of California at Irvine. The first possibility is that they were once very normal, very small galaxies with lots of normal and dark matter from which most of the star-making gases were blasted away by the death throes of their first generation of stars, themselves made of primordial hydrogen and helium of the Big Bang.

The second possibility is that the dwarf galaxies never drew much of the primordial gases in the first place. So they never had enough of the stuff to make more stars than we see today.

Neither the numbers of dwarfs or how they were made would seem very important in the larger scheme of things, except for all that dark matter they carry around and the inexplicable ways they carry it.

For instance, said Bullock, "All of these galaxies have exactly the same mass. We don’t know that that means."

Finding the answer to this and other looming dark matter mysteries will ultimately go a long way towards working out the details about how our Milky Way and other galaxies came into being.