2016ApJ...819L..20B

Ultra-diffuse galaxies (UDGs) have the sizes of giants but the luminosities of dwarfs. A key to understanding their origins comes from their total masses, but their low surface brightnesses (µ(V) ≥ 25.0) generally prohibit dynamical studies. Here, we report the first such measurements for a UDG (VCC 1287 in the Virgo cluster), based on its globular cluster system dynamics and size. From seven GCs we measure a mean systemic velocity v_sys = 1071_–15⁺¹⁴ km s^–1, thereby confirming a Virgo cluster association. We measure a velocity dispersion of 33_–10⁺¹⁶ km s^–1 within 8.1 kpc, corresponding to an enclosed mass of (4.5 ± 2.8) x 10⁹M_☉ and a g-band mass-to-light ratio of ([?\text?]M/L)_g=106_–54⁺¹²⁶ within an effective radius. From the cumulative mass curve, along with the GC numbers, we estimate a virial mass of ∼8 x 10¹⁰M_☉, yielding a dark-to-stellar mass fraction of ∼3000. We show that this UDG is an outlier in M_star-M_halo relations, suggesting extreme stochasticity in relatively massive star-forming halos in clusters. Finally, we discuss how counting GCs offers an efficient route to determining virial masses for UDGs.