What's in a name? Quantifying the interplay between definition,
  orientation and shape of UDGs using the ROMULUS Simulations
  We explore populations of ultra-diffuse galaxies (UDGs) in isolated,
satellite, and cluster environments using the Romulus25 and RomulusC
simulations, including how the populations vary with UDG definition and viewing
orientation. Using a fiducial definition of UDGs, we find that isolated UDGs
have notably larger semi-major (b/a) and smaller semi-minor (c/a) axis ratios
than their non-UDG counterparts, i.e., they are more oblate, or diskier. This
is in line with previous results that adopted the same UDG definition and
showed that isolated UDGs form via early, high spin mergers. However, the
choice of UDG definition can drastically affect what subset of a dwarf
population are classified as UDGs, changing the number of UDGs by up to ~45% of
the dwarf population. We also find that a galaxy's classification as a UDG is
dependent on its viewing orientation, and this dependence decreases as
environmental density increases. Overall, we conclude that some definitions for
UDG used in the literature manage to isolate a specific formation mechanism for
isolated dwarfs, while less restrictive definitions erase a link to formation
mechanism. Thus, how we define UDG populations must be considered if we want to
understand the formation and evolution of UDGs.
