For the first time in a decade, astronomers have found new dwarf galaxies -- ones with just billions of stars or even less compared with the hundreds of billions in our own -- orbiting the Milky Way. And they've found nine of them. That's the most that have ever turned up at once. The findings were published Tuesday in the Astrophysical Journal.
The new dwarfs are a billion times dimmer than the Milky Way and a million times less massive, the researchers who discovered them report. They were found near the Large and Small Magellanic Cloud, which are the two biggest dwarf galaxies orbiting the Milky Way. The closest of these nine newly found objects is less than 100,000 light years away, but the most distant is more than 1 million light years off. The objects were found using data recovered by the Dark Energy Survey, a five-year effort to photograph a large portion of the southern sky in unprecedented detail supported by over 120 scientists around the world. In fact, two separate research groups made the discovery independently using the data, and released their reports jointly.
The team is actually only certain that three of the objects are indeed dwarf galaxies. The rest might be globular clusters, which are groups of stars not held together by dark matter like galaxies are. Instead, globular clusters of stars are kept in place by the gravity of a galaxy they orbit. But it's the dark matter that astronomers are most interested in. We can't see dark matter, but scientists are pretty sure that it makes up more than a quarter of the universe's mass. Dark matter is really just our term for the matter we know is there -- the stuff that takes up space not held by better understood objects, such as stars and planets -- but don't yet understand the properties or behavior of. We can observe its gravitational pull, keeping galaxies together and separate from each other, but that's about it.
Unlike larger galaxies bursting with stars, dwarf galaxies have way more dark matter than normal matter. So for astronomers studying dark matter, they're basically an all-you-can-eat buffet.
"The discovery of so many satellites in such a small area of the sky was completely unexpected," Sergey Koposov of Cambridge's Institute of Astronomy, the Cambridge study's lead author, said in a statement. "I could not believe my eyes."
"Dwarf satellites are the final frontier for testing our theories of dark matter," Vasily Belokurov of the Institute of Astronomy, one of the study's co-authors, added. "We need to find them to determine whether our cosmological picture makes sense. Finding such a large group of satellites near the Magellanic Clouds was surprising, though, as earlier surveys of the southern sky found very little, so we were not expecting to stumble on such treasure."
The closest object is so close to the Milky Way that it's being torn apart by the larger galaxy's pull. The most distant is still on the outer edges of our own galaxy, but it's close to being pulled into the cosmic tug-of-war that will destroy it.