Reconciling mass-estimates of ultra-diffuse galaxies

The virial masses of ultra-diffuse galaxies (UDGs) have been estimated using the kinematics and abundance of their globular cluster populations, leading to disparate results. Some studies conclude that UDGs reside in massive dark matter halos while others, controversially, argue for the existence of UDGs with no dark matter at all. Here we show that these results arise because the uncertainties of these mass estimates have been substantially underestimated. Indeed, applying the same procedure to the well-studied Fornax dwarf spheroidal would conclude that it has an "overmassive" dark halo or, alternatively, that it lacks dark matter. We corroborate our argument with self-consistent mocks of tracers in cosmological halos, showing that masses from samples with $5 < N < 10$ tracers (assuming no measurement errors) are uncertain by at least an order of magnitude. Finally, we estimate masses of UDGs with HST imaging in Coma and show that their recent mass measurements (with adequate uncertainties) are in agreement with that of other dwarfs, such as Fornax.. We also provide bias and scatter factors for a range of sample sizes and measurement errors, of wider applicability.