An overmassive Dark Halo around an Ultra-diffuse Galaxy in the Virgo Cluster

Abstract

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 ($\mu (V)\ge$ 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 $v_{\mathrm{sys}}$ = 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 $(4.5\pm 2.8)\times 10^9$ $M_{\odot }$ 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 $\sim 8\times 10^{10}$ $M_{\odot }$, yielding a dark-to-stellar mass fraction of $\sim 3000$. We show that this UDG is an outlier in $M_{\mathrm{star}}-M_{\mathrm{halo}}$ 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.

Accepted for publication in ApJ letters