Clues to the nature of ultradiffuse galaxies from estimated galaxy velocity dispersions

We describe how to estimate the velocity dispersions of ultradiffuse galaxies (UDGs) using a previously defined galaxy scaling relationship. The method is accurate for the two UDGs with spectroscopically measured dispersions, as well as for ultracompact galaxies, ultrafaint galaxies, and stellar systems with little or no dark matter. This universality means that the relationship can be applied without further knowledge or prejudice regarding the structure of a galaxy. We then estimate the velocity dispersions of UDGs drawn from two published samples and examine the distribution of total masses. We find, in agreement with the previous studies of two individual UDGs, that these systems are dark matter dominated systems, and that they span a range of at least 10¹⁰< M₂₀₀/M_⊙<10¹². These galaxies are not, as an entire class, either all dwarfs or all failed L_∗galaxies. Estimates of the velocity dispersions can also help identify interesting subsets of UDGs, such as those that are likely to have the largest mass-to-light ratios, for subsequent spectroscopic study.