Do Invisible Galaxies Swirl Around the Milky Way?

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Unless you're some kind of space nut, you're probably entirely unaware that the Milky Way, our home galaxy, isn't traveling through the cosmos alone. It's shadowed by a litter of nearly a dozen dwarf galaxies that are far smaller and dimmer — a pack of cubs, you might say, yapping around the edges of the mama grizzly. If you widen your gaze to include a much larger swath of space, you'll find what's called the Local Group — a half-dozen full-size galaxies and about 30 dwarfs.

But even hardcore space people may not know that there's something very wrong with this picture. According to cosmological theory, says MIT astrophysicist Simona Vegetti, "there should be thousands of dwarf galaxies in the Local Group." That's because the earliest days of the cosmos were not a tidy time, and after the big galaxies came into being, a lot of debris ought to have been left behind — "debris," in this case, meaning little galaxies made partly of what's known as cold dark matter. The fact that we don't see the galaxies, she says, is due to one of three things: Either they're simply too faint to detect, or there's something unusual about the local cosmic neighborhood that would explain why it departs from the larger rule. Or — and this is the troubling alternative — maybe the theory itself, which has been generally accepted for the past 30 years or so, is fundamentally wrong in some way.

The likeliest answer is the first one — that the galaxies are there but we're just not seeing them. If so, Vegetti and several colleagues have taken a major step toward flushing them out of hiding. In the current issue of Nature, the group reports having found a pipsqueak of a galaxy with a mass equal to about 113 million suns (the Milky Way contains more than 100 billion stars) located halfway across the visible universe — and they've done it using an imaginative technique for capturing and refining images of very faint, very distant objects. What worked for a cosmic formation a full 10 billion light-years away could surely work for small, dark galaxies much closer to home.

To spot the distant dwarf, the astronomers first turned to one of the twin Keck Telescopes in Hawaii; with mirrors 33 ft. (11 m) across, each of the Kecks qualifies as one of the most powerful light-gathering machines in the world. That wasn't enough, though: the scientists had to sharpen the telescope's vision with an advanced adaptive-optics system, which erases the blurring caused by Earth's atmosphere.

But even that wouldn't cut it. "You won't be able to see this dwarf galaxy directly," says Vegetti, "without something like the European Extremely Large Telescope." That giant, slated for first light in the early 2020s, will have a mirror more than 130 ft. (40 m) across, with 16 times the Keck's light-gathering power. Unwilling to sit on their hands for a decade, Vegetti and her team relied instead on a cosmic magnification effect called gravitational lensing, in which the gravity of one galaxy bends and distorts the light from another galaxy lying far beyond it.

When they applied this technique and examined the distorted image that swam into view, they could tell it was created not just by the gravity of a galaxy alone but by a galaxy with a little extra bump on one side — a tiny dwarf, hanging on to its very edge. It isn't the first time the lensing method has been used to find a dwarf galaxy, but there are only a couple of other examples, and this one is by far the most distant — and the least massive.

So far, astronomers don't have enough examples of faraway dwarfs to say anything meaningful about the overall population of the species. It's like trying to predict an election result after exit-polling just three voters. Still, says Vegetti, "it's consistent with what we would expect" — which is to say, cosmological theory isn't in any obvious danger yet.

But even if all the dwarfs that ought to be swarming around the Milky Way are indeed there, that doesn't mean they aren't in some kind of peril. Astronomers believe that the Milky Way grew to its present size by swallowing small satellite galaxies that ventured too close — and this cannibalism is still going on. If there really are thousands of dwarf galaxies loitering quietly in our cosmic neighborhood, the Milky Way could be eating well for billions of years to come.