Globular Cluster Candidates in a Subaru Hyper Suprime-Cam (HSC) survey of the M81 group

We use archival Subaru Hyper Suprime-Cam (HSC) data in concert with space-based GALEX, IRAC and Gaia DR2 data to select candidate Globular clusters (GCs) in the M81 group for future study and follow-up. We use a small sample of previously-discovered GCs to tune our selection criteria, finding that bright already-known GCs in the M81 group have sizes that are typically slightly larger than the (typically ~0.6-0.7”) Subaru PSF in our fields. For a subset of GCs that are compact and bright enough, Gaia DR2 gives confirmation of their spatial extent in the form of a large difference between its reported G-band magnitude and the total G-band inferred from the combined BP and RP fluxes. In the Subaru optical g, r and i-bands, GCs appear to have colors that are slightly different from stars, but by relatively little. The inclusion of space-based GALEX (for a bright subset of clusters) and archival IRAC data yields dramatic improvements in color separation, as the long wavelength baseline aids somewhat in the separation from stars, and clearly separates GCs from many compact background galaxies. All of these selections have ambiguity, and we discuss the pros and cons of each type of selection, and give insight into the nature of the candidates via visual inspection of Subaru images.