Comparing the properties of local globular cluster systems: implications for the formation of the Galactic halo

Abstract

(Abridged) We investigate the hypothesis that some fraction of the globular clusters presently observed in the Galactic halo formed in external dwarf galaxies, by means of a detailed comparison between Zinn's "old halo", "young halo", and "bulge/disk" sub-systems and the globular clusters in local dwarf galaxies. The Galactic sub-systems possess distinct spatial and age distributions, age-metallicity relationships, and orbital parameters, although we observe some OH clusters with ages and orbits more similar to those of YH objects. Almost all Galactic globulars with large core radii (r_c) fall into the YH sub-system, while the OH and BD ensembles are characterized by compact clusters. The majority of the external clusters are indistinguishable from the Galactic YH objects in terms of HB morphology, but 20-30% of external clusters have HB morphologies most similar to the Galactic OH clusters. In addition, the external clusters have an r_c distribution which very closely matches that for the YH objects. The old halo r_c distribution can be very well represented by a composite distribution formed from 83-85% of objects with structures typical of BD clusters, and 15-17% of objects with structures typical of external clusters. Taken together our results fully support the accretion hypothesis. We conclude that all 30 YH clusters and 15-17% of the OH clusters (10-12 objects) are of external origin. The Galaxy may therefore have experienced ~7 merger events with cluster-bearing dSph-type galaxies during its lifetime, building up 45-50% of the mass of the stellar halo. Finally, we identify a number of OH objects which have properties characteristic of accreted clusters. Several of the clusters associated with the recently proposed CMa dwarf fall into this category.