A Rich Globular Cluster System in Dragonfly 17: Are Ultra-Diffuse
  Galaxies Pure Stellar Halos?
  Observations of nearby galaxy clusters at low surface brightness have
identified galaxies with low luminosities, but sizes as large as L* galaxies,
leading them to be dubbed "ultra-diffuse galaxies" (UDGs). The survival of UDGs
in dense environments like the Coma cluster suggests that UDGs could reside in
much more massive dark halos. We report the detection of a substantial
population of globular clusters (GCs) around a Coma UDG, Dragonfly 17 (DF17).
We find that DF17 has a high GC specific frequency of S_N=26+/-13. The GC
system is extended, with an effective radius of 12"+/-2", or 5.6+/-0.9 kpc at
Coma distance, 70% larger than the galaxy itself. We also estimate the mean of
the GC luminosity function to infer a distance of 97 (+17/-14) Mpc, providing
redshift-independent confirmation that one of these UDGs is in the Coma
cluster. The presence of a rich GC system in DF17 indicates that, despite its
low stellar density, star formation was intense enough to form many massive
star clusters. If DF17's ratio of total GC mass to total halo mass is similar
to those in other galaxies, then DF17 has an inferred total mass of ~10^11
solar masses, only ~10% the mass of the Milky Way, but extremely dominated by
dark matter, with M/L_V~1000. We suggest that UDGs like DF17 may be "pure
stellar halos", i.e., galaxies that formed their stellar halo components, but
then suffered an early cessation in star formation that prevented the formation
of any substantial central disk or bulge.
