Do ultra diffuse galaxies with rich globular clusters systems have overly massive haloes?

ABSTRACT Some Ultra Diffuse Galaxies (UDGs) appear to host exceptionally rich globular cluster (GC) systems compared to normal galaxies of the same stellar mass. After re-examining these claims, we focus on a small sample of UDGs from the literature that have both rich GC systems (NGC 20) and a measured galaxy velocity dispersion. We find that UDGs with more GCs have higher dynamical masses and that GC-rich UDGs are dark matter dominated within their half-light radii. We extrapolate these dynamical masses to derive total halo masses assuming cuspy and cored mass profiles. We find reasonable agreement between halo masses derived from GC numbers (assuming the GC number–halo mass relation) and from cored halo profiles. This suggests that GC-rich UDGs do not follow the standard stellar mass–halo mass relation, occupying overly massive cored haloes for their stellar mass. A similar process to that invoked for some Local Group dwarfs, of early quenching, may result in GC-rich UDGs that have failed to form the expected mass of stars in a given halo (and thus giving the appearance of overly an massive halo). Simulations that correctly reproduce the known properties of GC systems associated with UDGs are needed.