Probing the Microscopic with the Macroscopic: from Properties of Star Cluster Systems to Properties of Cluster-Forming Regions

TL;DR

This paper explores how the properties of star cluster systems reflect the physics of cluster formation, emphasizing the role of the mass-radius relation of cluster-forming regions and its implications for cluster evolution and star formation thresholds.

Contribution

It demonstrates that the mass-radius relation of cluster-forming regions influences cluster infant weight-loss and supports a volume density threshold for star formation, linking molecular cloud properties to star cluster characteristics.

Findings

CFRg with constant mean volume density explains the invariant young cluster mass function slope.

A volume density threshold drives star formation, affecting cluster and molecular cloud properties.

The mass function of star clusters is steeper than that of molecular clouds due to dissociation of properties.

Abstract

To understand how systems of star clusters have reached their presently observed properties constitutes a powerful probe into the physics of cluster formation, without needing to resort to high spatial resolution observations of individual cluster-forming regions (CFRg) in distant galaxies. In this contribution I focus on the mass-radius relation of CFRgs, how it can be uncovered by studying the gas expulsion phase of forming star clusters, and what the implications are. I demonstrate that, through the tidal field impact upon exposed star clusters, the CFRg mass-radius relation rules cluster infant weight-loss in dependence of cluster mass. The observational constraint of a time-invariant slope for the power-law young cluster mass function is robustly satisfied by CFRgs with a constant mean volume density. In contrast, a constant mean surface density would be conducive to the preferential destruction of high-mass clusters. A purely dynamical line-of-reasoning leads therefore to a conclusion consistent with star formation a process driven by a volume density threshold. Developing this concept further, properties of molecular clumps and CFRgs naturally get dissociated. This allows to understand: (i) why the star cluster mass function is steeper than the molecular cloud (clump) mass function; (ii) the presence of a massive star formation limit in the mass-size space of molecular structures.