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

We investigate the hypothesis that some fraction of the globular clusters presently observed in the Galactic halo formed in external dwarf galaxies. This is done by means of a detailed comparison between the “old halo”, “young halo,” and “bulge/disk” sub-systems defined by Zinn (1993a) and the globular clusters in the LMC, SMC, and Fornax and Sagittarius dwarf spheroidal galaxies. We first use high quality photometry from Hubble Space Telescope images to derive a complete set of uniform measurements of horizontal branch (HB) morphology in the external clusters. We also compile structural and metallicity measurements for these objects and update the database of such measurements for the Galactic globular clusters, including new calculations of HB morphology for 11 objects. Using these data together with recent measurements of globular cluster kinematics and ages we examine the characteristics of the three Galactic cluster sub-systems. Each is quite distinct in terms of their spatial and age distributions, age-metallicity relationships, and typical orbital parameters, although we observe some old halo clusters with ages and orbits more similar to those of young halo objects. In addition, almost all of the Galactic globular clusters with large core radii fall into the young halo sub-system, while the old halo and bulge/disk ensembles are characterized by compact clusters. We demonstrate that the majority of the external globular clusters are essentially indistinguishable from the Galactic young halo objects in terms of HB morphology, but ∼ 20 − 30 per cent of external clusters have HB morphologies most similar to the Galactic old halo clusters. We further show that the external clusters have a distribution of core radii which very closely matches that for the young halo objects. The old halo distribution of core radii can be very well represented by a composite distribution formed from ∼ 83 − 85 per cent of objects with structures typical of bulge/disk clusters, and ∼ 15 − 17 per cent of objects with structures typical of external clusters. Taken together our results fully support the accretion hypothesis. We conclude that all 30 young halo clusters and 15− 17 per cent of the old halo clusters (10 − 12 objects) are of external origin. Based on cluster number counts, we estimate that the Galaxy may have experienced ∼ 7 merger events with cluster-bearing dwarf-spheroidal-type galaxies during its lifetime, building up ∼ 45−50 per cent of the mass of the Galactic stellar halo. Finally, we identify a number of old halo objects which have properties characteristic of accreted clusters. Several of the clusters associated with the recently proposed dwarf galaxy in Canis Major fall into this category.