ORIGINAL

Abstract

Ultra-diffuse galaxies (UDGs) are exceptionally large in size ($r_{\rm eff} > 1.5$ kpc) for their low stellar masses (${\sim} 10^{8}\,{\rm M}_\odot$). The UDG formation mechanism is uncertain, with models predicting a range of dark matter halo masses. UDGs may have dwarf-like halos with $M_{\rm halo}\approx10^{10-11}\,{\rm M}_\odot$, or Milky-Way-like halos which are at least a factor of ten larger. Halo mass is thought to be positively correlated with the globular cluster (GC) abundance of a galaxy, thus the GC populations of UDGs can provide insight into the UDG formation process. Previous work has suggested a large range of UDG GC abundances, but these results were heavily weighted towards UDGs in high density environments which contain large amounts of baryonic and dark matter. In this paper, we address this bias by studying a sample of ultra diffuse galaxies (UDGs) in low density environments. We use new {\it Hubble Space Telescope} observations of nine UDGs to study their GC populations. We find that while the bulk of our UDGs have GC abundances consistent with those in normal dwarf galaxies, two of these UDGs have possible excess GC populations. These two UDGs both have GC luminosity functions, or GC brightness distributions, consistent with those of normal galaxies and other UDGs. We then combine our nine objects with the previous works mentioned above to create a catalog of UDGs with characterized GC populations which spans a uniquely diverse range of environments. We use this catalog to examine broad properties in the GC populations of low stellar mass galaxies. We see a possible trend of increasing UDG GC abundance with density of environment, although confirmation will require more observations of UDGs in low density environments. Finally, in an analysis of well-defined UDG and dwarf galaxy samples, we find that GC abundance increases with stellar mass and is positively correlated with galaxy size at fixed stellar mass, but we see no significant stellar mass-galaxy size relation. These results may be in tension with the standard GC abundance-halo mass relation, and may suggest that the stellar-halo mass relation has large scatter at low stellar masses.