N-body Models of Extended Clusters

TL;DR

This study uses direct N-body simulations to explore how extended star clusters form and evolve in different galactic tidal environments, revealing that weak tidal fields and initial conditions are key factors.

Contribution

It demonstrates that extended clusters can naturally form in weak tidal fields with initial tidal filling, and supports the idea they are often accreted from disrupted dwarf galaxies.

Findings

Extended clusters can form in weak tidal fields if initially tidally filled.

Differences in initial tidal filling significantly affect cluster evolution and size.

Extended clusters are unlikely to form in strong tidal fields like M31's inner regions.

Abstract

We use direct N-body simulations to investigate the evolution of star clusters with large size-scales with the particular goal of understanding the so-called extended clusters observed in various Local Group galaxies, including M31 and NGC6822. The N-body models incorporate a stellar mass function, stellar evolution and the tidal field of a host galaxy. We find that extended clusters can arise naturally within a weak tidal field provided that the tidal radius is filled at the start of the evolution. Differences in the initial tidal filling-factor can produce marked differences in the subsequent evolution of clusters and the size-scales that would be observed. These differences are more marked than any produced by internal evolution processes linked to the properties of cluster binary stars or the action of an intermediate-mass black hole, based on models performed in this work and previous work to date. Models evolved in a stronger tidal field show that extended clusters cannot form and evolve within the inner regions of a galaxy such as M31. Instead our results support the suggestion many extended clusters found in large galaxies were accreted as members of dwarf galaxies that were subsequently disrupted. Our results also enhance the recent suggestion that star clusters evolve to a common sequence in terms of their size and mass.