The Properties of Bound and Unbound Molecular Cloud Populations Formed in Galactic Disc Simulations

TL;DR

This study uses galaxy-scale simulations to analyze how galactic environment influences molecular cloud properties, revealing differences between inner and outer disc clouds and their impact on star formation.

Contribution

It demonstrates that a mixed population of bound and unbound clouds can reproduce observed molecular cloud scaling relations and highlights environmental effects on cloud properties and star formation.

Findings

Outer disc clouds are smaller, less massive, and have lower velocity dispersions.

Galactic shear significantly influences cloud properties at large radii.

Star formation rates depend on the local surface density and environment.

Abstract

We explore the effect of galactic environment on properties of molecular clouds. Using clouds formed in a large-scale galactic disc simulation, we measure the observable properties from synthetic column density maps. We confirm that a significant fraction of unbound clouds forms naturally in a galactic disc environment and that a mixed population of bound and unbound clouds can match observed scaling relations and distributions for extragalactic molecular clouds. By dividing the clouds into inner and outer disc populations, we compare their distributions of properties and test whether there are statistically significant differences between them. We find that clouds in the outer disc have lower masses, sizes, and velocity dispersions as compared to those in the inner disc for reasonable choices of the inner/outer boundary. We attribute the differences to the strong impact of galactic shear on the disc stability at large galactocentric radii. In particular, our Toomre analysis of the disc shows a narrowing envelope of unstable masses as a function of radius, resulting in the formation of smaller, lower mass fragments in the outer disc. We also show that the star formation rate is affected by the environment of the parent cloud, and is particularly influenced by the underlying surface density profile of the gas throughout the disc. Our work highlights the strengths of using galaxy-scale simulations to understand the formation and evolution of cloud properties - and the star formation within them - in the context of their environment.