It’s not every day that you find a rare dwarf galaxy orbiting your own. It’s even more remarkable, then, to find nine of them. But that’s exactly what’s happened, according to teams of scientists from the U.S. Department of Energy’s Dark Energy Survey and Cambridge University. The findings present an unprecedented opportunity to study how dark matter shapes our universe.
Dwarf galaxies are so special because they’re unbelievably tiny on a galactic scale – they typically contain 100 stars or less. The Milky Way galaxy, on the other hand, is completely average in size, yet still contains billions of stars. That makes finding dwarf galaxies incredibly difficult, and it’s why finding nine of them in the same part of the sky is a major development.
“The discovery of so many satellites in such a small area of the sky was completely unexpected,” said Cambridge’s Institute of Astronomy’s Sergey Koposov, the Cambridge study’s lead author. “I could not believe my eyes.”
Dwarf galaxies are of particular interest to a program like the Dark Energy Survey, as they’re dark matter dominant. That is, they contain far more mass than we can see in the form of dark matter. Scientists aren’t entirely sure what constitutes dark matter, but it’s believed to consist of particles that annihilate each other and release gamma rays. Dwarf galaxies don’t have any other gamma ray generators, which makes them ideal specimens for studying dark matter.
“The large dark matter content of Milky Way satellite galaxies makes this a significant result for both astronomy and physics,” said Alex Drlica-Wagner of Fermilab, one of the leaders of the Dark Energy Survey analysis.
There’s still some degree of uncertainty – three of the newly discovered objects are definitely dwarf galaxies, while the other six could either be dwarfs or “globular clusters,” which are similar to dwarf galaxies but without the dark matter component. The closest of these is 97,000 light years away, while the furthest is 1.2 million light years away.
“Dwarf satellites are the final frontier for testing our theories of dark matter,” said Dr Vasily Belokurov of the Institute of Astronomy, one of the study’s co-authors. “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.”
Scientists expect to find as many as 30 dwarf galaxies in the same area as the study continues.