Tuesday, July 5, 2016

A Dark Galaxy Hosting ~100 Globular Clusters

Dragonfly 44 is 2nd largest ultra diffused galaxy (UDG) in the Coma cluster of galaxies, and the only one that has been spectroscopically confirmed to be a member of the Coma cluster. UDGs are a population of large, very low surface brightness, spheroidal-shaped galaxies. Observations of Dragonfly 44 indicate it has a dynamical mass of about 7 billion times the mass of the Sun within a half light radius of approximately 15,000 light years. A galaxy's half light radius is basically the size of the area centred on the galaxy that contributes to half the galaxy's overall brightness. A whopping 98 percent of the galaxy's mass within its half light radius is in the form of dark matter.

Dragonfly 44 is also accompanied by a large population of globular clusters. Images of Dragonfly 44 show a population of approximately 100 globular clusters. These globular clusters are distributed in a halo around Dragonfly 44. Based on the distribution of globular clusters, the total mass enclosed within the halo of Dragonfly 44 is estimated to be roughly one trillion times the mass of the Sun, making Dragonfly 44 similar in mass to the Milky Way. Even though Dragonfly 44 is similar in mass to the Milky Way, its abundance of stars is ~100 times less than the Milky Way. The night sky from a hypothetical planet in Dragonfly 44 would appear much emptier of stars.

van Dokkum et al. (2016), "A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra Diffuse Galaxy Dragonfly 44", arXiv:1606.06291 [astro-ph.GA]