In an unexpected finding, an international team of astronomers have discovered a massive galaxy that consists almost entirely of dark matter – an unidentified form of matter that, scientists predict, accounts for around 27% of the universe's total mass.
No one is exactly sure what dark matter is, even though it is essential to the stability of the universe, as well as our understanding of physics.
Dragonfly 44, as the galaxy has been named, was spotted using two of the world's most powerful telescopes, the WM Keck Observatory and the Gemini North Telescope both located on Mauna Kea, a dormant volcano in Hawaii. The findings have been published in The Astrophysical Journal Letters.
Dragonfly 44 escaped the gaze of astronomers for decades as it is very faint. But last year, the first signs of something strange were noticed when the Dragonfly Telephoto Array identified a galaxy with so few stars that, according to what we know of galaxies, it should break apart. Something had to be holding it together.
So the team used Keck and Gemini to take a closer look at the stars in Dragonfly 44 to see if they could reveal any clues. "Motions of the stars tell you how much matter there is," Pieter van Dokkum, an author of the study, said. "They don't care what form the matter is, they just tell you that it's there. In the Dragonfly galaxy, stars move very fast. So there was a huge discrepancy. Using Keck Observatory, we found many times more mass indicated by the motions of the stars, then there is mass in the stars themselves."
Despite the galaxy being about the same mass as our galaxy, the Milky Way, only one hundredth of 1% of its matter is in the form of stars and other normal matter because most of its mass is comprised of dark matter.
"We have no idea how galaxies like Dragonfly 44 could have formed," Roberto Abraham, a co-author of the study, said. "The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we're just guessing."
"This has big implications for the study of dark matter," van Dokkum said. "It helps to have objects that are almost entirely made of dark matter so we don't get confused by stars and all the other things that galaxies have. The only such galaxies we had to study before were tiny. This finding opens up a whole new class of massive objects that we can study."
"Ultimately what we really want to learn is what dark matter is," van Dokkum said. "The race is on to find massive dark galaxies that are even closer to us than Dragonfly 44, so we can look for feeble signals that may reveal a dark matter particle."