Ultra-diffuse galaxies ( UDGs ) are unusual galaxies with low luminosities , similar to classical dwarf galaxies , but sizes up to \sim 5 larger than expected for their mass .
Some UDGs have large populations of globular clusters ( GCs ) , something unexpected in galaxies with such low stellar density and mass .
We have carried out a comprehensive study of GCs in both UDGs and classical dwarf galaxies at comparable stellar masses using HST observations of the Coma cluster .
We present new imaging for 33 Dragonfly UDGs with the largest effective radii ( > 2 kpc ) , and additionally include 15 UDGs and 54 classical dwarf galaxies from the HST/ACS Coma Treasury Survey and the literature .
Out of a total of 48 UDGs , 27 have statistically significant GC systems , and 11 have candidate nuclear star clusters .
The GC specific frequency ( S _ { N } ) varies dramatically , with the mean S _ { N } being higher for UDGs than for classical dwarfs .
At constant stellar mass , galaxies with larger sizes ( or lower surface brightnesses ) have higher S _ { N } , with the trend being stronger at higher stellar mass .
At lower stellar masses , UDGs tend to have higher S _ { N } when closer to the center of the cluster , i.e. , in denser environments .
The fraction of UDGs with a nuclear star cluster also depends on environment , varying from \sim 40 % in the cluster core , where it is slightly lower than the nucleation fraction of classical dwarfs , to \lesssim 20 \% in the outskirts .
Collectively , we observe an unmistakable diversity in the abundance of GCs , and this may point to multiple formation routes .