Characterising the Globular Cluster Systems of Three Local Group Dwarf Galaxies: NGC 6822, NGC 147 and NGC 185

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Abstract We present deep HST photometry for 26 globular clusters (GCs) residing in three Local Group dwarf galaxies: the isolated dwarf irregular (dIrr) galaxy, NGC 6822, and the M31 dwarf elliptical (dE) satellites, NGC 147 and NGC 185. From their colour-magnitude diagrams (CMDs), we quantify their red giant branch (RGB) and horizontal branch (HB) morphologies, and employ new empirical relationships to derive measurements of metallicity, reddening and distance. Additionally, we measure sizes and V-band magnitudes from their integrated light profiles. We find that the clusters span a range of metallicities, from [Fe/H] ~−0.7 to ≲ −2; however, the three dwarfs have very similar mean GC metallicities (~−1.7 dex) despite their very different evolutionary histories. In contrast, we find that almost all of NGC 6822’s GCs exhibit red HB morphologies, whereas those in the two dEs are predominantly blue. We highlight three outlying GCs in NGC 6822 that have very low metallicities yet very red HBs; they are also the most extended clusters in our sample, with half-light radii of ~13 − 17 pc. If these clusters are young, their origin is difficult to explain given their very remote locations, and metallicities which are significantly lower than those of the old and intermediate-age stars in NGC 6822. These three clusters are strikingly similar to several GCs linked to substructure in the outer halo of M31, suggesting either the recent accretion of an NGC 6822-like dwarf by M31, or that both M31 and NGC 6822 have recently accreted a similar low-mass system. Additionally, we find compelling evidence that another NGC 6822 cluster, SC7, is tidally distorted.

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