Clues to the nature of ultra diffuse galaxies from estimated galaxy
  velocity dispersions
  We describe how to estimate the velocity dispersions of ultra diffuse
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 ultra compact galaxies, ultra faint 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 $10^{10} < M_{200}/M_\odot < 10^{12} $. 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.
