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Galaxy surprise sheds light on dark matter

A study of small galaxies circling around the Milky Way found that while they range dramatically in brightness, they all surprisingly pack about the same mass.
Image: Satellite galaxies
A map shows the major satellite galaxies surrounding the Milky Way.J. Bullock, M. Geha, R. Powell
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A study of small galaxies circling around the Milky Way found that while they range dramatically in brightness, they all surprisingly pack about the same mass. The work suggests there is a minimum size for galaxies, and it could shed light on mysterious dark matter.

Spinning around the Milky Way are at least 23 pint-sized galaxies, each shining with the light of anywhere from a thousand to a billion suns. Though each of these galaxies is very dim compared to large galaxies like our own, they span a large range in brightness.

Astronomers led by Louis Strigari of the University of California-Irvine studied the movements of individual stars in these satellite galaxies to determine the mass of each galaxy.

"What we found was astonishing, which was that they all had the same mass," said researcher James Bullock, a UC-Irvine astrophysicist. "It's not what we were expecting — we were really taken off guard."

Loaded with dark matter
The finding could help explain the mysterious stuff called dark matter and how it affects the formation of galaxies. Nobody knows what dark matter is, but its presence is revealed by gravity that is not produced by the regular matter that can be seen.

Despite their wide-ranging brightnesses, all of the 23 satellite galaxies around the Milky Way seem to have a central mass of 10 million times that of the sun. And what's more, almost all of that mass seems to be made up of dark matter, with just the tiniest smidgen of visible matter producing stars.

"These are by far the dimmest galaxies that have ever been discovered, and the least luminous of these things are the most dark-matter-dominated things that we know about," Bullock told

Though they qualify as galaxies, the satellites are not pinwheel spirals like the Milky Way and its cousins. Rather, these dwarf galaxies look more like diffuse, puffy balls of light.

Dark matter wells
The fact that these galaxies, the smallest ever seen, all weigh about the same may mean that there is a lower threshold for the mass of galaxies. Though as to why there would be a minimum galactic size, "I have to say right now we're totally at a loss," Bullock said.

The researchers do have some ideas, though.

For one, maybe there are no dark matter clumps smaller than these galaxies, and their size represents the critical mass necessary for dark matter to condense into a clump.

"Maybe we've kind of hit a limit of how dark matter can cluster, and if that's true, maybe that tells us something about the dark matter particle itself," Bullock said.

Another option is that dark matter can form smaller clumps than these, but it just can't give rise to visible-light galaxies, he said. Perhaps the process of galaxy formation, which isn't fully understood, depends on having a minimum mass to begin with.

"You can think of a dark matter clump as a well, and the more massive the dark matter clump, the deeper that well is, and the harder it is for the normal matter to float out of it," Bullock said. "It could be that there are smaller clumps, but their wells are so shallow that any normal matter just falls right out."

The hunt for dark matter
The astronomers want to do further research, such as a detailed study of the satellite galaxies around our neighbor, the Andromeda galaxy, to probe the meaning of their discovery.

"We want to try to figure out if what we're learning is really a limit on how massive a clump of dark matter can be, or just a limit on massive a galaxy can be," Bullock said.

The researchers hope that by combining their findings with new observations and predictions made by theoretical models of dark matter, scientists may ultimately get to the bottom of what dark matter is made of.

At the very least, the discovery raises many new questions.

"These are very intriguing results, absolutely," said Savvas Koushiappas, a physicist at Brown University who was not involved in the study. "The thing I find extremely puzzling is, well, what does this tell us about physics? Is it truly a problem with [galaxy formation], or is it touching something fundamental about dark matter? It's a very interesting result and deserves attention, and it's something that now we have to think about. We have a lot of work to do."

The scientists detail their findings in the Aug. 28 issue of the journal Nature.