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

Michael A. Beasley; Aaron J. Romanowsky; Vincenzo Pota; Ignacio Martín Navarro; David Martinez Delgado; Fabian Neyer; Aaron L. Deich

doi:10.3847/2041-8205/819/2/L20

paper

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

arXiv:1602.04002 · doi:10.3847/2041-8205/819/2/L20

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 ( $μ (V) \geq$ 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 $_{- 15}^{+ 14}$ 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 $(4.5 \pm 2.8) \times 1 0^{9}$ $M_{⊙}$ and a $g$ -band mass-to-light ratio of $(M / L)_{g} = 10 6_{- 54}^{+ 126}$ . From the cumulative mass curve, along with the GC numbers, we estimate a virial mass of $\sim 8 \times 1 0^{10}$ $M_{⊙}$ , yielding a dark-to-stellar mass fraction of $\sim 3000$ . 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.

arXiv commentAccepted for publication in ApJ letters

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

Abstract

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