2018MNRAS.480.1956C

We present the first measurement of the globular cluster (GC) population surrounding the elliptical galaxy J07173724+3744224 (z = 0.1546). This galaxy is located in the foreground in the field-of-view of the Hubble Space Telescope (HST) Frontier Fields observations of galaxy cluster MACS J0717.5+3745 (z = 0.5458). Based on deep HST Advanced Camera for Surveys (ACS) F435W, F606W, and F814W images, we find a total GC population of N_tot = 3441 ± 1416. Applying the appropriate extinction correction and filter transformation from ACS F814W to the Johnson V band, we determine that the host galaxy has an absolute magnitude of M_V = -22.2. The specific frequency was found to be S_N = 4.5 ± 1.8. The radial profile of the GC system was best fit using a power law of the form σ ∼ R^–0.6, with the GC population found to be more extended than the halo light of the host galaxy (σ_halo ∼ R^–1.7). The F435W - F814W colour distribution suggests a bimodal population, with red GCs 1-3 times more abundant than blue clusters. These results are consistent with the host elliptical galaxy J07173724+3744224 having formed its red metal-rich GCs in situ, with the blue metal-poor GCs accreted from low-mass galaxies.