AN OVERMASSIVE DARK HALO AROUND AN ULTRA-DIFFUSE GALAXY IN THE VIRGO CLUSTER

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 (mu(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 nu(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 10(9) M-circle dot and a g-band mass-to-light ratio of (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 similar to 8 x 10(10) M-circle dot, yielding a dark-to-stellar mass fraction of similar to 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.