Mergers, tidal interactions, and mass exchange in a population of disc globular clusters

TL;DR

This study uses N-body simulations to show that interactions and mergers among disc globular clusters significantly influence their internal structures and may explain observed metallicity spreads in Milky Way GCs.

Contribution

First comprehensive analysis demonstrating how mutual interactions and mergers shape the properties of disc globular clusters in a Milky Way-like galaxy.

Findings

Two major mergers observed in simulations

Clusters can develop multiple sub-populations

Interactions may explain metallicity spreads in GCs

Abstract

We present the results of a self-consistent $N$-body simulation following the evolution of a primordial population of thick disc globular clusters (GCs). We study how the internal properties of such clusters evolve under the action of mutual interactions, while they orbit a Milky Way-like galaxy. For the first time, through analytical and numerical considerations, we find that physical encounters between disc GCs are a crucial factor that contributed to the shape of the current properties of the Galactic GC system. Close passages or motion on similar orbits may indeed have a significant impact on the internal structure of clusters, producing multiple gravitationally bound sub-populations through the exchange of mass and even mergers. Our model produces two major mergers and a few small mass exchanges between pairs of GCs. Two of our GCs accrete stars from two companions, ending up with three internal sub-populations. We propose these early interactions and mergers between thick disc GCs with slightly different initial chemical compositions as a possible explanation for the presence of the spreads in metallicity observed in some of the massive Milky Ways GCs.