Ultra diffuse galaxies (UDGs) have the sizes of giant galaxies 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 7 GCs we measure a mean systemic velocity vsys = 1071−15+14 km/s, thereby confirming a Virgo-cluster association. We measure a velocity dispersion of 33−10+16 km/s within 8.1 kpc, corresponding to an enclosed mass of (4.5±2.8)×109M⊙ and a g-band mass-to-light ratio of (M/L)g=106−54+126. From the cumulative mass curve, along with the GC numbers, we estimate a virial mass of ∼8×1010M⊙, yielding a dark-to-stellar mass fraction of ∼3000. We show that this UDG is an outlier in Mstar−Mhalo 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.
@article{arxiv.1602.04002,
title = {An overmassive Dark Halo around an Ultra-diffuse Galaxy in the Virgo Cluster},
author = {Michael A. Beasley and Aaron J. Romanowsky and Vincenzo Pota and Ignacio Martín Navarro and David Martinez Delgado and Fabian Neyer and Aaron L. Deich},
journal= {arXiv preprint arXiv:1602.04002},
year = {2016}
}