Most galaxies are dominated by dark matter. Our own galaxy is about 85% dark matter by mass. But recently a galaxy was discovered that is almost entirely dark matter. Known as Dragonfly 44, less than 0.01% of its mass is regular matter.
Dark matter is highly mysterious due to the fact that we haven’t detected it directly. Since dark matter doesn’t interact with light, we can only observe its gravitational effect on light and the motions of stars. The elusive nature of dark matter has led some to propose other ideas such as modified gravity, but these don’t agree well with the Universe we observe. Given that dark matter is so mysterious, how do we know Dragonfly 44 is a dark matter galaxy?
To begin with the galaxy is very diffuse. It is only slightly smaller than the Milky Way in size, but has about 1% the number of stars. By itself that might not mean much, but Dragonfly 44 is part of a cluster of galaxies known as the Coma cluster. Over time such a diffuse galaxy would be torn apart by other nearby galaxies unless it had a large mass to hold it together. This is supported by the speeds of the stars within the galaxy. Stars within a galaxy generally orbit the center of a galaxy similar to the way planets orbit a star. The more mass a galaxy has, the faster the stars tend to orbit. By looking at the distribution of stellar speeds within a galaxy, we can get a measure of how much mass a galaxy has. Finally, Dragonfly 44 has about 100 globular clusters orbiting it. These are small but dense clusters of stars that generally form a kind of halo around larger galaxies. Typically, the larger the galaxy the more globular clusters it will have. All of this supports the idea that Dragonfly 44 has a mass about equal to that of our Milky Way. Given the low number of visible stars, that means 99.99% of its mass must be dark matter.
These kinds of Ultra Diffuse Galaxies (UDGs) could help us better understand dark matter. While galaxies such as the Milky Way are mostly composed of dark matter, their central regions where stars are most dense are dominated by regular matter. As a result it’s difficult to study the effects of dark matter in detail. With diffuse dark matter galaxies we can see the effects of dark matter more clearly. If dark matter does interact with itself to produce small amounts of light, as some have proposed, it might be visible within UDGs.
Paper: Pieter van Dokkum, et al. A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra Diffuse Galaxy Dragonfly 44. The Astrophysical Journal Letters, Volume 828, Number 1 (2016) arXiv:1606.06291 [astro-ph.GA]