Researchers have found a very distant dwarf galaxy, that is allmost entirely made up of dark-matter!

Galaxies like 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, a team of researchers including a 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 universe, and is by far the most distant.

The dwarf galaxy is a satellite, meaning that 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 appearing in the Jan. 18 online edition of 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% 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.

In the new study, Vegetti worked with her former PhD supervisor, Professor Leon Koopmans of the University of Groningen, Netherlands; David Lagattuta and Professor Christopher Fassnacht of the University of California at Davis; Matthew Auger of the University of California at Santa Barbara; and John McKean of the Netherlands Institute for Radio Astronomy.

The team turned to more distant galaxies to search for dark satellites, using a method called gravitational lensing. To use this technique, researchers find two galaxies aligned with each other, as viewed from Earth. The more distant galaxy emits light rays that are reflected by the closer galaxy (which acts as a lens). By analyzing
the patterns of light rays deflected by the foreground lens galaxy, the researchers can determine if there are any satellite galaxies clustered around it and measure how massive they are.

The researchers used the Keck Telescope in Hawaii to make theirobservations, taking advantage of a special piece of optical equipment that provides sharper images of the sky. They plan to use the samemethod to look for more satellite galaxies in other regions of the universe, which they believe could help corroborate or challenge predictions of how dark matter behaves.

“Now we have one dark satellite, but suppose that we don’t find enough of them - then we will have to change the properties of dark matter,” Vegetti says. “Or, we might find as many satellites as we see in the simulations, and that will tell us that dark matter has the properties we think it has.”

For example, because the temperature determines the mass and number of satellites that form, it may be necessary to adjust the current temperature estimates for dark matter if the number of dark satellites found is less than projected.

Written by Anne Trafton, MIT News Office