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Discovery of a New Class of Galaxies Challenges Our Understanding of How Galaxy Formation Works

Dragonfly 44 near the Coma cluster has the same mass as the Milky Way but only 1 percent of the visible matter

The dark galaxy Dragonfly 44 (Pieter van Dokkum, Roberto Abraham, Gemini, Sloan Digital Sky Survey)

Dragonfly 44, the dim galaxy about 300 million light years away, doesn’t have very many stars. That's one reason why it wasn’t located until last year when researchers turned their gaze on the edges of the Coma cluster, a large group of about 1,000 galaxies. “Very soon after its discovery, we realized this galaxy had to be more than meets the eye,” Yale astronomer Pieter van Dokkum says in a press release. “It has so few stars that it would quickly be ripped apart unless something was holding it together.”

The velocity of the stars within the galaxy are much higher than would be expected, which means Dragonfly 44 has a mass much greater than researchers can detect with their telescopes. That means the galaxy is likely composed primarily of dark matter, a theoretical substance that physicists believe makes up about 27 percent of the universe, outweighing visible matter—the substance that makes up things like ferrets, tomatoes and planets—by 6 to 1.

Rachel Feltman at The Washington Post reports that the new galaxy is about the size of our own Milky Way, but with about 1 percent of the stars. Yet the mass of the galaxy is about 1 trillion times the mass of our own sun, roughly equivalent to our own galaxy. That means 99.99 percent of Dragonfly 44 is composed of dark matter.

“It’s pretty crazy, the difference from the Milky Way is a factor of 100,” Dokkum tells Feltman. “That's just something we never knew could happen.”

The research team, which published their results in Astrophysical Journal Letters, first came across the Dragonfly 44 while using the Dragonfly Telephoto Array based at the University of Toronto. The array uses commercially available telephoto lenses with a few tweaks, which turn out to be better at spotting dim cosmic smudges than traditional telescopes. Hanneke Weitering at reports that researchers found 47 “diffuse” galaxies near Coma, the largest and most visible of which was Dragonfly 44. In fact, when they found Dragonfly 44, researchers weren’t sure if it was a celestial body or they were having a problem with their lenses.

They spent six nights at Hawaii’s W. M.​ Keck Observatory watching the new galaxy and taking measurements to determine that it was, in fact, real—and represented a new class of galaxy.

That means Dragonfly 44 is not just a one-off or a novelty. Dokkum tells Feltman that the discovery challenges our understanding of galaxy formation. “We thought that that ratio of matter to dark matter was something we understood. We thought the formation of stars was kind of related to how much dark matter there is, and Dragonfly 44 kind of turns that idea on its head,” he says. “It means we don’t understand, kind of fundamentally, how galaxy formation works.”

Now, Dokkum says, the race is on to find other galaxies composed of dark matter that are closer than Dragonfly 44. Dark matter is believed to give off a faint ultraviolet signal, so researchers hope a neighboring dark galaxy might provide the first direct evidence for the existence of the elusive substance.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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