Hyper Suprime-Cam Low Surface Brightness Galaxies II: A Hubble Space
  Telescope Study of the Globular Cluster Systems of Ultra-Diffuse Galaxies in
  Groups
  We increase the sample of ultra diffuse galaxies (UDGs) in lower density
environments with characterized globular cluster (GC) populations using new
Hubble Space Telescope observations of nine UDGs in group environments. While
the bulk of our UDGs have GC abundances consistent with normal dwarf galaxies,
two of these UDGs have excess GC populations. These two UDGs both have GC
luminosity functions consistent with higher surface brightness galaxies and
cluster UDGs. We then combine our nine objects with previous studies to create
a catalog of UDGs with analyzed GC populations that spans a uniquely diverse
range of environments. We use this catalog to examine broader trends in the GC
populations of low stellar mass galaxies. The highest GC abundances are found
in cluster UDGs, but whether cluster UDGs are actually more extreme requires
study of many more UDGs in groups. We find a possible positive correlation
between GC abundance and stellar mass, and between GC abundance and galaxy size
at fixed stellar mass. However, we see no significant stellar-mass galaxy-size
relation, over our limited stellar mass range. We consider possible origins of
the correlation between GC abundance and galaxy size, including the possibility
that these two galaxy properties are both dependent on the galaxy dark matter
halo, or that they are related through baryonic processes like internal
feedback.
