Stellar Velocity Dispersion and Dynamical Mass of the Ultra-Diffuse
  Galaxy NGC 5846_UDG1 from the Keck Cosmic Web Imager
  The ultra-diffuse galaxy in the NGC 5846 group (NGC 5846_UDG1) was shown to
have a large number of globular cluster (GC) candidates from deep imaging as
part of the VEGAS survey. Recently, Muller et al. published a velocity
dispersion, based on a dozen of its GCs. Within their quoted uncertainties, the
resulting dynamical mass allowed for either a dark matter free or a dark matter
dominated galaxy. Here we present spectra from KCWI which reconfirms membership
of the NGC 5846 group and reveals a stellar velocity dispersion for UDG1 of
$\sigma_{GC}$ = 17 $\pm$ 2 km/s. Our dynamical mass, with a reduced
uncertainty, indicates a very high contribution of dark matter within the
effective radius. We also derive an enclosed mass from the locations and
motions of the GCs using the Tracer Mass Estimator, finding a similar mass
inferred from our stellar velocity dispersion. We find no evidence that the
galaxy is rotating and is thus likely pressure-supported. The number of
confirmed GCs, and the total number inferred for the system ($\sim$45), suggest
a total halo mass of $\sim2 \times 10^{11}$ M$_{\odot}$. A cored mass profile
is favoured when compared to our dynamical mass. Given its stellar mass of
1.1$\times$10$^{8}$ M$_{\odot}$, NGC 5846_UDG1 appears to be an ultra-diffuse
galaxy with a dwarf-like stellar mass and an overly massive halo.
