The Formation and Evolution of Star Clusters and Galaxies

This paper addresses the questions of what we have learned about how and when dense star clusters form, and what studies of star clusters have revealed about galaxy formation and evolution. One important observation is that globular clusters are observed to form in galaxy mergers and starbursts in the local universe, which both provides constraints on models of globular cluster formation, and suggests that similar physical conditions existed when most early-type galaxies and their globular clusters formed in the past. A second important observation is that globular cluster systems typically have bimodal color distributions. This was predicted by merger models, and indicates an episodic formation history for elliptical galaxies. A third and very recent result is the discovery of large populations of intermediate age globular clusters in several elliptical galaxies through the use of optical to near-infrared colors. These provide an important link between young cluster systems observed in starbursts and mergers and old cluster systems. This continuum of ages of the metal-rich globular cluster systems also indicates that there is no special age or epoch for the formation of the metal-rich globular clusters, which comprise about half of the cluster population. The paper concludes with a brief discussion of recent results on the globular cluster low-mass X-ray binary connection. 1. Globular Cluster Formation A natural starting point for the discussion of galaxies and globular cluster systems is the formation of globular clusters (GCs). Any model of the formation of globular cluster systems and their host galaxies that does not include a consideration of how the globular clusters themselves form is necessarily incomplete. Fortunately, nature has provided us nearby examples of GC formation, most dramatically revealed in HST images (e.g. Whitmore et al. 1999, Zepf et al. 1999). The formation of globular clusters in galaxy mergers was predicted by Ashman & Zepf (1992) and Schweizer (1987) and then confirmed by many subsequent HST observations. Moreover, as discussed at this meeting by Bruce Elmegreen, these observations are providing an important testing ground for theoretical work on GC formation (e.g. Elmegreen 2002, Ashman & Zepf 2001).