The Lifecycle of Clusters in Galaxies

TL;DR

This paper reviews star cluster properties, their relation to host galaxy features, and how cluster formation and disruption processes vary with environment, highlighting observational and theoretical insights into cluster lifecycle dynamics.

Contribution

It provides a comprehensive synthesis of how galaxy environment influences star cluster formation efficiency and disruption, integrating observational data with theoretical predictions.

Findings

Cluster mass and luminosity functions follow power-laws with a slope of ~-2.

Environmental factors cause the steepening of these functions at high masses/luminosities.

Cluster formation efficiency varies with galaxy properties, especially star-formation rate density.

Abstract

We review many of the basic properties of star cluster systems, and focus in particular on how they relate to their host galaxy properties and ambient environment. The cluster mass and luminosity functions are well approximated by power-laws of the form $Ndm \propto M^{\alpha}dm$, with $\alpha\sim-2$ over most of the observable range. However, there is now clear evidence that both become steeper at high masses/luminosities, with the value of the downward turn dependent on environment. The host galaxy properties also appear to affect the cluster formation efficiency ($\Gamma$ - i.e., the fraction of stars that form in bound clusters), with higher star-formation rate density galaxies having higher $\Gamma$ values. Within individual galaxies, there is evidence for $\Gamma$ to vary by a factor of 3-4, likely following the molecular gas surface density, in agreement with recent predictions. Finally, we discuss cluster disruption and its effect on the observed properties of a population, focussing on the age distribution of clusters. We briefly discuss the expectations of theoretical and numerical studies, and also the observed distributions in a number of galaxies. Most observational studies now find agreement with theoretical expectations, namely nearly a constant cluster age distribution for ages up to ~100 Myr (i.e. little disruption), and a drastic steepening above this value caused by a combination of cluster disruption and incompleteness. Rapid cluster disruption for clusters with ages < 100 Myr is ruled out for most galaxies.