Selections from 2016: A Very Dark Galaxy

Editor’s note: In these last two weeks of 2016, we’ll be looking at a few selections that we haven’t yet discussed on AAS Nova from among the most-downloaded papers published in AAS journals this year. The usual posting schedule will resume after the AAS winter meeting.

A High Stellar Velocity Dispersion and ~100 Globular Clusters for the Ultra-Diffuse Galaxy Dragonfly 44

Published August 2016


Main takeaway:

Using the Keck Observatory and the Gemini North telescope in Hawaii, a team led by Pieter van Dokkum (Yale University) discovered the very dim galaxy Dragonfly 44, located in the Coma cluster. The team estimated the center of this galaxy’s disk to be a whopping 98% dark matter.

Why it’s interesting:

Dragonfly 44, though dim, was discovered to host around 100 globular clusters. Measuring the dynamics of these clusters allowed van Dokkum and collaborators to estimate the mass of Dragonfly 44: roughly a trillion times the mass of the Sun. This is similar to the mass of the Milky Way, and yet the Milky Way has over a hundred times more stars than this intriguing galaxy. It’s very unexpected to find a galaxy this massive that has a dark-matter fraction this high.

What we can learn from this:

How do ultra-faint galaxies like these form? One theory is that they’re “failed” normal galaxies: they have the sizes, dark-matter content, and globular cluster systems of much more luminous galaxies, but they were prevented from building up a normal stellar population. So far, Dragonfly 44’s properties seem consistent with this picture.


Pieter van Dokkum et al 2016 ApJL 828 L6. doi:10.3847/2041-8205/828/1/L6