The Dynamical Evolution of Accreted Star Clusters in the Milky Way

Abstract

We perform N-body simulations of star clusters in time-dependant galactic potentials. Since the Milky Way was built up through mergers with dwarf galaxies, its globular cluster population is made up of clusters formed both during the initial collapse of the Galaxy and in dwarf galaxies that were later accreted. Throughout a dwarf Milky Way merger, dwarf galaxy clusters are subject to a changing galactic potential. We investigate how this changing galactic potential affects the evolution of a cluster's half-mass radius. In particular, we simulate clusters on circular orbits around a dwarf galaxy that either falls into the Milky Way or evaporates as it orbits the Milky Way. We find that the dynamical evolution of a star cluster is determined by whichever galaxy has the strongest tidal field at the position of the cluster. Thus, clusters entering the Milky Way undergo changes in size as the Milky Way tidal field becomes stronger and that of the dwarf diminishes. We find that ultimately accreted clusters become the same size as a cluster born in the Milky Way on the same orbit. The change in size for accreted clusters occurs on a short time-scale, comparable to 1-2 cluster half-mass relaxation times. Assuming their initial sizes are similar, clusters born in the Galaxy and those that are accreted cannot be separated based on their current size alone.