The globular cluster systems in the coma ellipticals. II. Metallicity distribution and radial structure in NGC 4874, and implications for galaxy formation

Deep Hubble Space Telescope WFPC2 images in V and I are used to investigate the globular cluster system (GCS) in NGC 4874, the central cD galaxy of the Coma cluster. Although the luminosity function of the clusters displays its normal Gaussian-like shape and turnover level, other features of the system are surprising. We find the GCS to be (a) spatially extended, with core radius rc ∼ 22 kpc, (b) entirely metal poor (a narrow, unimodal metallicity distribution with <[Fe/H]> ∼ -1.5), and (c) modestly populated for a cD-type galaxy, with specific frequency SN = 3.7 ± 0.5. Model interpretations suggest to us that as much as half of this galaxy might have accreted from low-mass satellites, but no single one of the three classic modes of galaxy formation (accretion, disk mergers, in situ formation) can supply a fully satisfactory model for the formation of NGC 4874. Even when they are used in combination, strong challenges to these models remain. We suggest that the principal anomaly in this GCS is essentially the complete lack of metal-rich clusters. If these were present in normal (M87-like) numbers in addition to the metal-poor ones that are already there, then the GCS in total would more closely resemble what we see in many other giant E galaxies. This supergiant galaxy appears to have avoided forming globular clusters during the main metal-rich stage of star formation that built the bulk of the galaxy.