Accreted versus in situ Milky Way globular clusters

A large, perhaps dominant fraction, of the Milky Way’s halo is thought to be built-up by the accretion of smaller galaxies and their globular cluster (GC) systems. Here we examine the Milky Way’s GC system to estimate the fraction of accreted versus in situ formed GCs. We first assemble a high quality database of ages and metallicities for 93 Milky Way GCs from literature deep colour-magnitude data. The age-metallicity relation for the Milky Way’s GCs reveals two distinct tracks---one with near constant old age of ~12.8 Gyr and the other branches to younger ages. We find that the latter young track is dominated by globular clusters associated with the Sagittarius and Canis Major dwarf galaxies. Despite being…