The Formation Histories of Metal-Rich and Metal-Poor Globular Clusters

This review presents the results of ongoing studies of the formation histories of metal-poor and metal-rich globular clusters and their host galaxies. I first discuss the strong observational evidence that the globular cluster systems of most elliptical galaxies have bimodal metallicity distributions. I then focus on new results for metal-poor and metal-rich globular cluster systems. Metal-poor globular clusters are often associated with early structure formation, and I review new constraints on their formation epoch based on the ``bias'' of the number of metal-poor clusters with host galaxy mass. For metal-rich globular clusters, I discuss new results from ongoing optical to near-infrared photometric studies which both confirm an intermediate-age population in NGC 4365 and generally reveal a variety of formation histories for now quiescent ellipticals.