Intrinsic Morphology of Ultra-diffuse Galaxies
  With the published data of apparent axis ratios for 1109 ultra-diffuse
galaxies (UDGs) located in 17 low-redshift (z~ 0.020 - 0.063) galaxy clusters
and 84 UDGs in 2 intermediate-redshift (z~ 0.308 - 0.348) clusters, we take
advantage of a Markov Chain Monte Carlo approach and assume a ubiquitous
triaxial model to investigate the intrinsic morphologies of UDGs. In contrast
to the conclusion of Burkert (2017), i.e., the underlying shapes of UDGs are
purely prolate ($C=B<A$), we find that the data favor the oblate-triaxial
models ($C<B\lesssim A$) over the nearly prolate ones. We also find that the
intrinsic morphologies of UDGs are relevant to their stellar
masses/luminosities, environments, and redshifts. First, for the low-redshift
UDGs in the same environment, the more-luminous ones are always thicker than
the less-luminous counterparts, possibly due to the more voilent internal
supernovae feedback or external tidal interactions for the progenitors of the
more-luminous UDGs. The UDG thickness dependence on luminosity is distinct from
that of the typical quiescent dwarf ellipticals (dEs) and dwarf spheroidals
(dSphs) in the local clusters and groups, but resembles that of massive
galaxies; in this sense, UDGs may not be simply treated as an extension of the
dE/dSph class with similar evolutionary histories. Second, for the low-redshift
UDGs within the same luminosity range, the ones with smaller cluster-centric
distances are more puffed-up, probably attributed to tidal interactions.
Finally, the intermediate-redshift cluster UDGs are more flattened, which
plausibly suggests a `disky' origin for high-redshift, initial UDGs.
