If it does not kill them, it makes them stronger: collisional evolution of star clusters with tidal shocks

TL;DR

This paper explains the observed weak correlation between young star cluster radii and their masses as a result of the interplay between two-body relaxation and tidal shocks, affecting cluster evolution and the globular cluster mass function.

Contribution

It demonstrates that the mass-radius relation arises from a balance of relaxation and tidal shocks, challenging previous assumptions about cluster disruption and formation universality.

Findings

Clusters evolve towards a few parsecs radius regardless of initial size.

Low-mass clusters are more disrupted by GMC interactions.

High-redshift ISM conditions challenge universal globular cluster formation theories.

Abstract

The radii of young (<100 Myr) star clusters correlate only weakly with their masses. This shallow relation has been used to argue that impulsive tidal perturbations, or `shocks', by passing giant molecular clouds (GMCs) preferentially disrupt low-mass clusters. We show that this mass-radius relation is in fact the result of the combined effect of two-body relaxation and repeated tidal shocks. Clusters in a broad range of environments including those like the solar neighbourhood evolve towards a typical radius of a few parsecs, as observed, independent of the initial radius. This equilibrium mass-radius relation is the result of a competition between expansion by relaxation and shrinking due to shocks. Interactions with GMCs are more disruptive for low-mass clusters, which helps to evolve the globular cluster mass function (GCMF). However, the properties of the interstellar medium in high-redshift galaxies required to establish a universal GCMF shape are more extreme than previously derived, challenging the idea that all GCs formed with the same power-law mass function.