Astrophile: How to spot a dark-matter galaxy

Object type: Dwarf galaxy
Mass: 190,000,000 times the mass of the sun
Composition: Mostly dark matter

If we could don dark matter glasses and look at the universe around us, we might see thousands of miniature galaxies swarming about the luminous spirals that make up the Milky Way and Andromeda.

We can't ? but we have the next best thing. A technique known as gravitational lensing has allowed one of these dark dwarfs to be glimpsed, suggesting the Milky Way isn't as lonely as it looks to us Earthlings.

Astronomers think that galaxies usually grow by devouring smaller nearby clusters of stars called dwarf galaxies, no bigger than 100 million times the mass of the sun. According to this theory, the Milky Way and all other full-size galaxies should keep company with thousands of dwarfs. However, only 30 such companions have been spotted in our neighbourhood.

Where are all the missing minis hiding? One explanation is that they're mostly made of dark matter, the mysterious, aloof substance thought to make up 83 per cent of the mass in the universe but which is reluctant to interact with regular matter.

"They are there, but we just don't see them," says Simona Vegetti of the Massachusetts Institute of Technology.

Warped lens

Some of the Milky Way's known satellites, such as the Sagittarius dwarf galaxy, do seem to be mostly dark matter, hosting just 100 or fewer visible stars. It's hard to know how many more are lurking undercover, though. And it's especially hard to know if distant galaxies host any dark satellites at all.

"When you're trying to test theories using the Milky Way, at a certain point you have to ask, is the Milky Way a special place or not?" Vegetti says. "It's important to test in other galaxies."

Now, Vegetti and colleagues have tracked down a dark dwarf galaxy orbiting a massive elliptical galaxy 10 billion light years away. The invisible entity, part of a system called JVAS B1938+666, weighs in at about 190 million times the mass of the sun, making it similar in size to the Sagittarius dwarf.

The team found the dark galaxy by looking at the way its gravity warped the space-time around it, a technique called gravitational lensing. When the dwarf's vast companion passed in front of an even more distant galaxy, its gravity made the background galaxy look stretched out in a ring of light. Normally these rings, called Einstein rings, form nearly perfect circles. But the presence of the otherwise invisible dwarf galaxy introduced a small flaw, making the further galaxy look like a perfect china bowl with a chip in it.

Purely dark

"The great thing about the technique is that we can use it to detect satellite galaxies that are purely dark matter," says study co-author Chris Fassnacht of the University of California, Davis ? although it's hard to tell from here whether the galaxy has a handful of stars or not.

This is the second dark galaxy to be spotted outside our galactic system, but the first that is as small as the Milky Way's known companions. The other galaxy was just 2.6?billion light years away and 18 times as massive.

"These two together are telling us that it's not something special to have a satellite that small," Vegetti says. The universe may be swarming with small, dark galaxies, she says, though she and her team will have to keep looking to know for sure.

"We've found two, so everything looks promising," she says.

Journal reference: Nature, DOI: 10.1038/nature10669

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