Globular clusters in galaxy halos

Perhaps the most noteworthy of recent findings in extragalactic globular cluster research are the bimodal globular cluster metallicity distributions seen in massive early-type galaxies. We find that the metal-rich globular clusters are closely coupled to the galaxy and share a common chemical enrichment history with the galaxy field stars. The current data also indicate that the high SN galaxies have proportionately more metalpoor globular clusters, per unit galaxy light, than low SN galaxies. The gaseous merger model of Ashman & Zepf (1992) has difficulty explaining these trends, whereas they appear compatible with a multiphase collapse scenario. In this scenario, the metal-poor globular clusters are formed at an early stage in the collapse of the protogalactic cloud. The metal-rich globular clusters formed out of more enriched gas, roughly contemporaneously with the galaxy stars. In this sense the metal-rich globular clusters in elliptical galaxies are the analog of the metal-poor halo globular clusters in spirals. The disk globular clusters in spirals may represent a third phase of this formation process.