The Dynamical Evolution of Accreted Star Clusters in the Milky Way

TL;DR

This study uses N-body simulations to explore how the evolving galactic potential during dwarf galaxy mergers influences the size and evolution of accreted star clusters in the Milky Way.

Contribution

It demonstrates that the size evolution of accreted clusters is primarily governed by the dominant tidal field during galaxy mergers.

Findings

Accreted clusters quickly match the size of native Milky Way clusters on similar orbits.

Cluster size evolution is driven by the strongest tidal field at their location.

Initial sizes of clusters are not distinguishable after accretion.

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. Building on our previous work, 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 quickly become the same size as a cluster born in the Milky Way on the same orbit. 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.