Dim nearby galaxy is nearly 100% dark matter

Pieter van Dokkum, Roberto Abraham, Gemini Observatory/AURA

Dim nearby galaxy is nearly 100% dark matter

Astronomers have spotted a dim and unexpectedly dense galaxy that may be almost entirely made of dark matter. Dubbed Dragonfly 44, this nearby group of stars (yellowish smudge at center of right image) was discovered just last year and apparently has less than 1% the number of stars in our Milky Way galaxy. But the motions of stars within Dragonfly 44 (as inferred from images captured in an exposure lasting more than 33 hours and collected over 6 nights) tell a different story. Those stars are orbiting the center of the galaxy much faster than expected based on the estimated heft of its visible matter, suggesting Dragonfly 44 as a whole is more massive than meets the eye and is thus chock-full of dark matter. In fact, the group of stars contains an estimated mass of 1 trillion suns (about the same as our Milky Way) and is about 99.99% dark matter, the researchers report online today in Astrophysical Journal Letters. Dragonfly 44, which lies about 300 million light-years from Earth in the constellation of Coma Berenices, is part of the so-called Coma Cluster of galaxies (partially shown in the left panel above). By further analyzing a massive object that’s so predominantly made of dark matter, the team suggests, researchers may be able to figure out what the elusive stuff actually is and where it’s most likely to be found.