Reconciling mass estimates of ultradiffuse galaxies

The virial masses of ultradiffuse 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 haloes while van Dokkum et al., 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 haloes, showing that masses from samples with 5 < N < 10 kinematic tracers (assuming no measurement errors) are uncertain by at least an order of magnitude. Finally, we estimate masses of UDGs with Hubble Space Telescope imaging in Coma and show that their recent mass measurements (with adequate uncertainties) are in agreement with those 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.