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Monthly Notices of the Royal Astronomical Society







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We examine ultra-diffuse galaxies (UDGs) and their relation to non-UDGs in mass–radius–luminosity space. We begin by publishing Keck/KCWI spectroscopy for the Coma cluster UDG Y358, for which we measure both a recessional velocity and velocity dispersion. Our recessional velocity confirms association with the Coma cluster and Y358’s status as a UDG. From our velocity dispersion (19 ± 3 km s−1), we calculate a dynamical mass within the half-light radius, which provides evidence for a core in Y358’s dark matter halo. We compare this dynamical mass, along with those for globular cluster (GC)-rich/-poor UDGs in the literature, to mass profiles for isolated, gas-rich UDGs, and UDGs in the NIHAO/FIRE simulations. We find GC-poor UDGs have dynamical masses similar to isolated, gas-rich UDGs, suggesting an evolutionary pathway may exist between the two. Conversely, GC-rich UDGs have dynamical masses too massive to be easily explained as the evolution of the isolated, gas-rich UDGs. The simulated UDGs match the dynamical masses of the GC-rich UDGs. However, once compared in stellar mass–halo mass space, the FIRE/NIHAO-simulated UDGs do not match the halo masses of either the isolated, gas-rich UDGs or the GC-rich UDGs at the same stellar mass. Finally, we supplement our data for Y358 with other UDGs that have measured velocity dispersions in the literature. We compare this sample to a wide range of non-UDGs in mass–radius–luminosity space, finding UDGs have a similar locus to non-UDGs of similar luminosity with the primary difference being their larger half-light radii.

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National Aeronautics and Space Administration


dynamics, galaxies: elliptical and lenticular, cD, galaxies: formation, galaxies: fundamental parameters, galaxies: haloes, galaxies: kinematics


This article originally appeared in Monthly Notices of the Royal Astronomical Society, Volume 518, Issue 3, 2023. The article can also be found online at:


Physics and Astronomy