A “Failed” Milky Way that is 99.99% Dark Matter

The galaxy Dragonfly 44, observed using GMOS-North, in wide field (left) and close-up (right) views. Dragonfly 44 is very faint for its mass and consists almost entirely of dark matter.

Astronomers have discovered a massive galaxy that is almost entirely dark matter. The galaxy, called Dragonfly 44, has very low surface brightness and was only discovered in 2014. New observations using the Gemini Multi-Object Spectrograph (GMOS) on Gemini North and the Keck II telescope (also on Maunakea) reveal the physical properties of the galaxy. They show that it is like a “failed” Milky Way, in having similar total mass, size, and population of globular clusters, lacking only the stars. Considering the entire galaxy, Pieter van Dokkum (Yale University) and collaborators estimate that 99.99% of the massive (10¹² M_Sun) galaxy is dark matter. The Gemini press release has more information, including links to high-resolution images from this Fast Turnaround program, and full results are published in The Astrophysical Journal Letters.