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.
