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

Abstract

ABSTRACT 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 ( &mgr; ( 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) × 109 M ⊙ 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 ∼8 × 1010 M ⊙, 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.