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Observations of nearby galaxy clusters at low surface brightness haveidentified galaxies with low luminosities, but sizes as large as L* galaxies,leading them to be dubbed "ultra-diffuse galaxies" (UDGs). The survival of UDGsin dense environments like the Coma cluster suggests that UDGs could reside inmuch more massive dark halos. We report the detection of a substantialpopulation 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 GCsystem is extended, with an effective radius of 12"+/-2", or 5.6+/-0.9 kpc atComa distance, 70% larger than the galaxy itself. We also estimate the mean ofthe GC luminosity function to infer a distance of 97 (+17/-14) Mpc, providingredshift-independent confirmation that one of these UDGs is in the Comacluster. The presence of a rich GC system in DF17 indicates that, despite itslow stellar density, star formation was intense enough to form many massivestar clusters. If DF17's ratio of total GC mass to total halo mass is similarto those in other galaxies, then DF17 has an inferred total mass of ~10^11solar masses, only ~10% the mass of the Milky Way, but extremely dominated bydark matter, with M/L_V~1000. We suggest that UDGs like DF17 may be "purestellar halos", i.e., galaxies that formed their stellar halo components, butthen suffered an early cessation in star formation that prevented the formationof any substantial central disk or bulge.

A Rich Globular Cluster System in Dragonfly 17: Are Ultra-Diffuse Galaxies Pure Stellar Halos?