The Size of Star Clusters Accreted by the Milky Way

TL;DR

This study uses N-body simulations to show that star clusters accreted by the Milky Way quickly adapt their size to the galactic environment, making their structure indistinguishable from native clusters over time.

Contribution

It demonstrates that the half mass radius of accreted clusters rapidly adjusts to the Milky Way's tidal field, challenging the use of size as an identifier for accreted clusters.

Findings

Cluster size responds within 1-2 relaxation times.

Size changes depend on the tidal field strength.

Core radius remains unaffected by galactic potential.

Abstract

We perform N-body simulations of a cluster that forms in a dwarf galaxy and is then accreted by the Milky Way to investigate how a cluster's structure is affected by a galaxy merger. We find that the cluster's half mass radius will respond quickly to this change in potential. When the cluster is placed on an orbit in the Milky Way with a stronger tidal field the cluster experiences a sharp decrease in size in response to increased tidal forces. Conversely, when placed on an orbit with a weaker tidal field the cluster expands since tidal forces decrease and stars moving outwards due to internal effects remain bound at further distances than before. In all cases, we find that the cluster's half mass radius will eventually be indistinguishable from a cluster that has always lived in the Milky Way on that orbit. These adjustments occur within 1-2 half mass relaxation times of the cluster in the dwarf galaxy. We also find this effect to be qualitatively independent of the time that the cluster is taken from the dwarf galaxy. In contrast to the half mass radius, we show the core radius of the cluster is not affected by the potential the cluster lives in. Our work suggests that structural properties of accreted clusters are not distinct from clusters born in the Milky Way. Other cluster properties, such as metallicity and horizontal branch morphology, may be the only way to identify accreted star clusters in the Milky Way.