New globular cluster candidates in the M81 group

Abstract

The study of outer halo globular cluster (GC) populations can give insight into galaxy merging, globular cluster accretion and the origin of GCs. We use archival Subaru Hyper Suprime-Cam (HSC) data in concert with space-based GALEX, IRAC and Gaia EDR3 data to select candidate Globular clusters (GCs) in the outer halo of the M81 group for confirmation and future study. 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 Subaru PSF in our fields. In the optical bands, GCs appear to have colours that are only slightly different from stars. The inclusion of archival IRAC data yields dramatic improvements in colour separation, as the long wavelength baseline aids somewhat in the separation from stars and clearly separates GCs from many compact background galaxies. We show that some previously spectroscopically-identified GCs in the M81 group are instead foreground stars or background galaxies. GCs close to M82 have radial velocities suggesting that they fell into the M81 group along with M82. The overall M81 GC luminosity function is similar to the Milky Way and M31. M81’s outer halo GCs are similar to the Milky Way in their metallicities and numbers, and much less numerous than M31’s more metal-rich outer halo GC population. These properties reflect differences in the three galaxies’ merger histories, highlighting the possibility of using outer halo GCs to trace merger history in larger samples of galaxies.

Is the M81 Fast Radio Burst Host Globular Cluster Special?

“…In this section we compare a few dynamical and structural parameters measured for the M81 FRB host GC with those measured for Galactic clusters and GC candidates in M81 (with measurements from NH11; 85 classified as "confirmed" and 125 classified as "good" candidates by NH11). We note that Pan et al (2022) et al suggest that the HST selected candidates of NH11 have an estimated contamination rate of 8% for the bright clusters.…”

“…M81ʼs GC system has been extensively studied with photometry and spectroscopy in the optical and near-infrared (Perelmuter & Racine 1995;Ma et al 2007;Nantais et al 2011;Chies-Santos et al 2022;Pan et al 2022). The X-ray source population associated with the GCs has also been well characterized (Hunt et al 2023).…”

“…The FRB GC is located roughly 19.6′ from the center of M81, which translates to ≈20.5 kpc at the relevant distance. Pan et al (2022) used multiwavelength archival data and other information from the literature to assemble an accurate list of the GCs in M81 and included this object in their catalog. Based on an initial assessment of these various measurements, the FRB GC does not seem to be unusual, despite playing host to an extremely mysterious and energetic radio signal.…”