By Joshua Sokol
Among the thousand-plus galaxies in the Coma cluster, a massive clump of matter some 300 million light-years away, is at least one—and maybe a few hundred—that shouldn’t exist.
Dragonfly 44 is a dim galaxy, with one star for every hundred in our Milky Way. But it spans roughly as much space as the Milky Way. In addition, it’s heavy enough to rival our own galaxy in mass, according to results published in the Astrophysical Journal Letters at the end of August. That odd combination is crucial: Dragonfly 44 is so dark, so fluffy, and so heavy that some astronomers believe it will either force a revision of our theories of galaxy formation or help us understand the properties of dark matter, the mysterious stuff that interacts with normal matter via gravity and not much else. Or both.
The discovery came almost by accident. The astronomers Pieter van Dokkum of Yale University and Roberto Abraham of the University of Toronto were interested in testing theories of how galaxies form by searching for objects that have been invisible to even the most advanced telescopes: faint, wispy and extended objects in the sky. So their team built the Dragonfly Telephoto Array, a collection of modified Canon lenses that focus light onto commercial camera sensors. This setup cut down on any scattered light inside the system that might hide a dim object.
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