Tiny Galaxies Shed New Light on Dark Matter

Anne Minard
for National Geographic News
August 27, 2008
The nearly two dozen dwarf galaxies known to orbit the Milky Way vary greatly in brightness, from a thousand times to ten million times the luminosity of the sun.

But at their cores, all of them have about the same mass, according to new work that sheds light on how the enigmatic substance known as dark matter helps galaxies form.

This mystery matter is thought to make up the majority of mass in the universe but is invisible except for its gravitational influence on ordinary matter.

Using the relative speeds of stars in the Milky Way's satellite galaxies, a team led by Louis Strigari of the University of California, Irvine (UCI) found that 18 of the 23 known satellite galaxies have a common central mass of about ten million times that of the sun.

That means dark matter is the most likely source of the unexpected mass in the faint galaxies. It also suggests that dark matter can provide the minimum mass required for galaxies to form.

"We've gone down to the smallest galaxies we can see," said Manoj Kaplinghat, a UCI astrophysicist and study co-author.

"What's surprising is there's so much dark matter, even though these guys are little. They barely have a few thousand stars."

Dark Milky Way

Research over the past two decades has revealed that dark matter provides structure throughout the universe.

Scientists believe that galaxies form as dark matter's gravity attracts normal matter, creating the well-known groupings of gases, stars, and other objects. They also suspect that small galaxies merge over time to create larger galaxies such as our Milky Way.

"Basically galaxies like our own wouldn't have formed if we didn't have dark matter," Kaplinghat said.

Currently the ratio of dark matter to visible material in the Milky Way is about ten to one, he added.

(Related: "New Milky Way Map Created; Shows Fewer Main Arms" [June 3, 2008].)

"The bulk of the Milky Way is dark," he said. "It's true for any galaxy we've looked at, even bigger things, like clusters of galaxies."

But the tiny dwarf galaxies are even more chock full of dark matter, with ratios near ten thousand to one.

Since these galaxies have roughly the same mass no matter how bright they are, the study authors say, the dark matter must be what supplies them with the critical mass needed for them to hold their shape as galaxies.

The paper appears in this week's issue of the journal Nature.

Exciting Times

Mario Mateo, an astronomer at the University of Michigan who was not involved in the new analysis, called the results "very interesting."

But, he points out, more work will need to be done to confirm the results.

The dwarf galaxies' close proximity to the Milky Way could affect the speed at which their stars move and thus the estimates of their masses, he said.

"A number of the folks on this paper, myself, and others are looking forward to new discoveries of other faint galaxies in other parts of the sky," he said. "These will help extend the results discussed in this paper."

For example, most of the Milky Way's dwarf galaxies were discovered as part of the Sloan Digital Sky Survey, a privately funded mission to map the universe.

But the ongoing survey currently covers only half of the sky as seen from Earth's Northern Hemisphere, leaving room for more satellite galaxies to be found once the southern sky is imaged.

Yesterday NASA unveiled early results from its newly named Fermi Gamma-ray Space Telescope, which could help astronomers pinpoint the nature of the particles that make up dark matter.

Scientists also hope to learn about dark matter's subatomic properties when the Large Hadron Collider, the world's largest particle accelerator, becomes operational later this year. (See a photo of the collider's final magnet being lowered into place last April.)

All of these missions will test prevailing theories about dark matter, study co-author Kaplinghat said, making it an exciting time to be an astrophysicist. "If you don't find something [about dark matter] in the next five to ten years," he said, "there's something very wrong with all the theories we have."

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