Last week, astronomers announced the discovery of a galaxy that is almost entirely comprised of dark matter. At first glance, the galaxy, Dragonfly 44, is roughly the same size as the Milky Way, but only contains 1 percent as many stars. Naively, one would expect this galaxy to be torn apart by its surroundings in the Coma Cluster. However, Dragonfly 44 is composed of 99.99% dark matter, which provides enough mass to hold its constituents together.
Data from the Sloan Digital Sky Survey and Gemini Telescope were used to study Dragonfly 44. The total mass of the galaxy was determined by observing the velocity distribution of its stars, and counting the number of surrounding globular clusters. Faster stars and more globular clusters are both indicative of a more massive galaxy. By subtracting the amount of normal matter in observed stars from the total measured mass, astronomers were able to directly estimate the amount of dark matter that makes up the galaxy.
Other galaxies have been observed with a similar dark matter composition to Dragonfly 44, but they were many orders of magnitude less massive. Dragonfly 44 is unique because no one has a clear idea as to how the galaxy formed. Researchers plan to use the same techniques to study an ensemble of similar galaxies. With enough statistics, astronomers might be able to resolve the mystery of its origin.
For many, the ultimate goal of these studies is to observe dark matter interactions. The centers of most galaxies are dominated by normal matter, making the study of dark matter difficult to accomplish. If scientists could find a nearby galaxy like Dragonfly 44, both large and dark enough, there is some chance of observing interactions or signals that reveal clues to a dark matter particle.
Original Research: “A High Stellar Velocity Dispersion and 100 Globular Clusters for the Ultra Diffuse Galaxy Dragonfly 44” Astrophysical Journal Letters
Media Coverage:
“Scientists Discover a ‘Dark’ Milky Way” Yale News
“Newly Discovered Galaxy is Almost Entirely Made of Dark Matter” Forbes Science
Managing Correspondent: Karri Folan DiPetrillo