New Galaxy Is the Same Mass as the Milky Way, but 99.99% Dark Matter

If the brilliant stars and elegant spiral arms of the Milky Way define the prototypical galaxy, then Dragonfly 44 might be its near opposite.

A dim blob in the Coma Cluster some 321 million light years away, the newly described galaxy has only one-hundredth the number of stars as the Milky Way. Instead of stellar mass holding it all together, a massive amount of dark matter—99.99% of the galaxy’s mass—does the job.

dragonfly-44
The dark galaxy Dragonfly 44 seen on the left, at a wide angle, and right, at a close angle, as a dim, yellow blob of stars.

“The existence of nearly dark objects with this mass is unexpected,” write Yale astronomer Pieter van Dokkum and his colleagues in their new study published in Astrophysical Journal Letters.

The galaxy is so unusual that it might force astrophysicists to rethink their theories of how the large bodies form. Here’s Shannon Hall, reporting for New Scientist:

“It’s hard to argue with the observations, yet the conclusion from this paper runs counter to my understanding of how galaxies are formed,” says Marla Geha at Yale University. “I’m hoping these objects are rather rare and/or only form in special environments such as a dense galaxy cluster. Otherwise we may need to rewrite galaxy formation.”

Van Dokkum and his team used the 10-meter Keck II telescope based in Hawaii to measure the speed of the stars inside the galaxy and thus its mass. Faster speeds mean a more massive galaxy, which would normally imply a larger number of stars. But the relative paucity of stellar masses within Dragonfly 44 suggests that something else—dark matter—provides the gravitational glue that keeps it from flying apart.

The proportion of dark matter in Dragonfly 44 (99.99%) narrowly edges out the previous record holder, VCC 1287—also discovered by van Dokkum—which is 99.96% dark matter.