2016ApJ...828L...6V

Recently a population of large, very low surface brightness, spheroidal galaxies was identified in the Coma cluster. The apparent survival of these ultra-diffuse galaxies (UDGs) in a rich cluster suggests that they have very high masses. Here, we present the stellar kinematics of Dragonfly 44, one of the largest Coma UDGs, using a 33.5 hr integration with DEIMOS on the Keck II telescope. We find a velocity dispersion of σ={47}_{-6}^{+8}km {s}{-1}, which implies a dynamical mass of {M}_dyn(r_{1/2})={0.7}{-0.2}^{+0.3}×{10}^{10} {M}_☉ within its deprojected half-light radius of {r}_{1/2}=4.6±0.2 kpc. The mass-to-light ratio is M/L_{I}(r_{1/2})={48}{-14}^{+21} {M}_☉/L_☉, and the dark matter fraction is 98% within {r}_{1/2}. The high mass of Dragonfly 44 is accompanied by a large globular cluster population. From deep Gemini imaging taken in 0′′ /
. 4 seeing we infer that Dragonfly 44 has {94}_{-20}^{+25} globular clusters, similar to the counts for other galaxies in this mass range. Our results add to other recent evidence that many UDGs are "failed" galaxies, with the sizes, dark matter content, and globular cluster systems of much more luminous objects. We estimate the total dark halo mass of Dragonfly 44 by comparing the amount of dark matter within r=4.6 kpc to enclosed mass profiles of NFW halos. The enclosed mass suggests a total mass of ∼{10}^{12}{M}_☉, similar to the mass of the Milky Way. The existence of nearly dark objects with this mass is unexpected, as galaxy formation is thought to be maximally efficient in this regime.