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 ($\mu(V) \geq$ 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 $v_sys$
= 1071$^+14_-15$ km/s, thereby confirming a Virgo-cluster association. We
measure a velocity dispersion of 33$^+16_-10$ km/s within 8.1 kpc,
corresponding to an enclosed mass of $(4.5 2.8)\times10^9$ $M_ødot$
and a $g$-band mass-to-light ratio of $(M/L)_g = 106^+126_-54$. From the
cumulative mass curve, along with the GC numbers, we estimate a virial mass of
$\sim8\times10^10$ $M_ødot$, yielding a dark-to-stellar mass fraction of
$\sim3000$. We show that this UDG is an outlier in $M_star - M_\rm
halo$ 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.
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