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Spectroscopic Confirmation of the Existence of Large, Diffuse Galaxies in the Coma Cluster

We recently identified a population of low surface brightness objects in the field of the z=0.023 Coma cluster, using the Dragonfly Telephoto Array. Here we present Keck spectroscopy of one of the largest of these "ultra-diffuse galaxies" (UDGs), confirming that it is a member of the cluster. The galaxy has prominent absorption features, including the Ca II H+K lines and the G-band, and no detected emission lines. Its radial velocity of cz=6280 +- 120 km/s is within the 1 sigma velocity dispersion of the Coma cluster. The galaxy has an effective radius of 4.3 +- 0.3 kpc and a Sersic index of 0.89 +- 0.06, as measured from Keck imaging. We find no indications of tidal tails or other distortions, at least out to a radius of ~2 r_e. We show that UDGs are located in a previously sparsely populated region of the size - magnitude plane of quiescent stellar systems, as they are ~6 magnitudes fainter than normal early-type galaxies of the same size. It appears that the luminosity distribution of large quiescent galaxies is not continuous, although this could largely be due to selection effects. Dynamical measurements are needed to determine whether the dark matter halos of UDGs are similar to those of galaxies with the same luminosity or to those of galaxies with the same size.