Globular Cluster Systems in Brightest Cluster Galaxies: Bimodal Metallicity Distributions and the Nature of the High‐Luminosity Clusters

We present new (B,I) photometry for the globular cluster systems in eightBrightest Cluster Galaxies (BCGs), obtained with the ACS/WFC camera on theHubble Space Telescope. In the very rich cluster systems that reside withinthese giant galaxies, we find that all have strongly bimodal colordistributions All the BCGs show population gradients, with much higher relativenumbers of red clusters within 5 kpc of their centers, consistent with theirhaving formed at later times than the blue, metal-poor population. A strikingnew feature of the color distributions emerging from our data is that for thebrightest clusters (M_I < -10.5) the color distribution becomes broad and lessobviously bimodal. we suggest that it may be a characteristic of many BCGs.Furthermore, the blue (metal-poor) clusters become progressively redder withincreasing luminosity, following a mass/metallicity scaling relation Z ~M^0.55. We argue that these GCS characteristics are consistent with ahierarchical-merging formation picture in which the metal-poor clusters formedin protogalactic clouds or dense starburst complexes with gas masses in therange 10^7 - 10^10 M_Sun, but where the more massive clusters on average formedin bigger clouds with deeper potential wells where more pre-enrichment couldoccur.