The role of galactic dynamics in shaping the physical properties of giant molecular clouds in Milky Way-like galaxies

TL;DR

This study uses high-resolution simulations to explore how large-scale galactic dynamics influence the physical and kinematic properties of giant molecular clouds in Milky Way-like galaxies, revealing correlations with galactic shear and gravitational stability.

Contribution

It provides the first detailed analysis linking galactic dynamical environments to the properties and behaviors of molecular clouds through advanced simulations.

Findings

Molecular clouds are highly overdense and over-pressured compared to the ambient medium.

Cloud properties like elongation and angular momentum correlate with galactic shear and gravitational free-fall rates.

HI clouds show broader dynamical correlations due to their lower pressure and density.

Abstract

We examine the role of the large-scale galactic-dynamical environment in setting the properties of giant molecular clouds in Milky Way-like galaxies. We perform three high-resolution simulations of Milky Way-like discs with the moving-mesh hydrodynamics code Arepo, yielding a statistical sample of $\sim 80,000$ giant molecular clouds and $\sim 55,000$ HI clouds. We account for the self-gravity of the gas, momentum and thermal energy injection from supernovae and HII regions, mass injection from stellar winds, and the non-equilibrium chemistry of hydrogen, carbon and oxygen. By varying the external gravitational potential, we probe galactic-dynamical environments spanning an order of magnitude in the orbital angular velocity, gravitational stability, mid-plane pressure and the gradient of the galactic rotation curve. The simulated molecular clouds are highly overdense ($\sim 100\times$) and over-pressured ($\sim 25\times$) relative to the ambient interstellar medium. Their gravo-turbulent and star-forming properties are decoupled from the dynamics of the galactic mid-plane, so that the kpc-scale star formation rate surface density is related only to the number of molecular clouds per unit area of the galactic mid-plane. Despite this, the clouds display clear, statistically-significant correlations of their rotational properties with the rates of galactic shearing and gravitational free-fall. We find that galactic rotation and gravitational instability can influence their elongation, angular momenta, and tangential velocity dispersions. The lower pressures and densities of the HI clouds allow for a greater range of significant dynamical correlations, mirroring the rotational properties of the molecular clouds, while also displaying a coupling of their gravitational and turbulent properties to the galactic-dynamical environment.