A Two-Phase Chemical Enrichment Model for the Milky Way Globular Cluster System

Many globular cluster systems have a distinct bimodal metallicity distribution function (MDF), which has strikingly similar features in many large galaxies of all types. By using the Milky Way cluster system as a typical example, we show that bimodal MDFs can be very well matched with a double "accreting-box" chemical enrichment model in which both the halo (metal-poor) and bulge (metal-rich) clusters form during an early phase of gas inflow simultaneously with star formation. However, differences in effective yield between the two phases are not enough by themselves to reproduce the observed MDF shape: gas infall is required for both phases, and either the initial gas or the infalling gas must have very different metallicities in the two separate phases.