Galaxies such as our own Milky Way are believed to form over billions of years through the coming together of many smaller galaxies. As a result, it is expected that there should be many smaller dwarf galaxies scattered around the Milky Way. However, very few of these tiny relic galaxies have been observed, which has led astronomers to conclude that many of them must have very few stars or may be made almost exclusively of dark matter.
In a discovery announced Jan. 18, 2013, a team of researchers including an MIT postdoc has found a dark dwarf galaxy about 10 billion light years from Earth. It is only the second such galaxy ever observed outside our local region of the universe, and is by far the most distant.
The newly discovered dwarf galaxy is a satellite, meaning it clings to the edges of a larger galaxy. “For several reasons, it didn’t manage to form many or any stars, and therefore it stayed dark,” says Simona Vegetti, a Pappalardo Fellow in MIT’s Department of Physics and lead author of a paper on the work in Nature.
Scientists theorize the existence of dark matter to explain observations that suggest there is far more mass in the universe than can be seen. They believe that dark matter should comprise about 25 percent of the universe; however, because the particles that make up dark matter do not absorb or emit light, they have so far proven impossible to detect and identify.
Computer modeling suggests that the Milky Way should have about 10,000 satellite galaxies, but only 30 have been observed. “It could be that many of the satellite galaxies are made of dark matter, making them elusive to detect, or there may be a problem with the way we think galaxies form,” Vegetti says.
“The existence of this low-mass dark galaxy is just within the bounds we expect if the universe is composed of dark matter that has a cold temperature. However, further dark satellites will need to be found to confirm this conclusion,” says Vegetti.
Andrey Kravtsov, associate professor of astronomy and astrophysics, says the new study is a “very valuable contribution” to the ongoing testing of the prediction that small clumps of dark matter should be found scattered around the edges of large galaxies. “The uncertainties are still quite large, but so far the abundance of such clumps is in accord with expectations of structure formation models based on cold dark matter scenario,” says Kravtsov, who was not involved in the research.