The Nature and Nurture of Star Clusters

TL;DR

This paper explores the hierarchical formation and structure of star clusters, their connection to star formation processes, and the evolution of globular cluster mass functions across different galactic environments.

Contribution

It provides a comprehensive analysis of the hierarchical nature of star clusters and proposes explanations for the observed properties and mass functions of globular clusters.

Findings

Star clusters exhibit hierarchical patterns in space and time.

The fraction of star formation in clusters varies with interstellar pressure.

Globular clusters have lost low-mass members, affecting their mass function.

Abstract

Star clusters have hierarchical patterns in space and time, suggesting formation processes in the densest regions of a turbulent interstellar medium. Clusters also have hierarchical substructure when they are young, which makes them all look like the inner mixed parts of a pervasive stellar hierarchy. Young field stars share this distribution, presumably because some of them came from dissolved clusters and others formed in a dispersed fashion in the same gas. The fraction of star formation that ends up in clusters is apparently not constant, but may increase with interstellar pressure. Hierarchical structure explains why stars form in clusters and why many of these clusters are self-bound. It also explains the cluster mass function. Halo globular clusters share many properties of disk clusters, including what appears to be an upper cluster cutoff mass. However, halo globulars are self-enriched and often connected with dwarf galaxy streams. The mass function of halo globulars could have initially been like the power law mass function of disk clusters, but the halo globulars have lost their low mass members. The reasons for this loss are not understood. It could have happened slowly over time as a result of cluster evaporation, or it could have happened early after cluster formation as a result of gas loss. The latter model explains best the observation that the globular cluster mass function has no radial gradient in galaxies.