Meet Dragonfly 44: The Milky Way’s Dark-Matter Twin

Astronomers have discovered something strange about an ultra-diffuse galaxy in the Coma Cluster called Dragonfly 44. It went largely overlooked by astronomers until recently, due to it only emitting 1 percent of the light that the Milky Way does, hence its "ultra diffuse" status. But Dragonfly 44 has recently been found to be nearly the same mass as our own galaxy, due to being made nearly entirely of dark matter.

Using the W. M. Keck Observatory and the Gemini North telescope, perched atop Mauna Kea in Hawai’i, researchers tracked the speed of the few stars that are visible in Dragonfly 44, to estimate the galaxy’s mass — the faster the stars orbit around the galaxy, the more massive the galaxy is as a whole. But instead of group of slow stars, the researchers found them to be moving at a speed that indicative of a stellar neighborhood that had a mass similar to our own galaxy’s.

“Motions of the stars tell you how much matter there is,” explains Yale University’s Pieter van Dokkum. “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.”

With the stars that are visible only accounting for about one percent of Dragonfly 44’s mass, this led to the conclusion that the rest of the galaxy must be made up of the elusive phenomena called dark matter, a hypothesized form of matter that neither emits, nor reflects light.

“This has big implications for the study of dark matter,” van Dokkum continues. “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. 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.”