The Importance of Tidal Forces in Molecular Cloud Dynamics

TL;DR

This study highlights the significant influence of tidal forces from the galactic environment on molecular cloud dynamics, demonstrating that external gravitational effects can alter cloud boundedness assessments and are crucial for accurate modeling.

Contribution

It introduces a method to incorporate external tidal forces into the analysis of molecular cloud boundedness, revealing their impact on cloud stability beyond internal gravity alone.

Findings

Tidal forces can unbind or bind clouds contrary to their apparent virial status.

Including external gravitational potential provides a more accurate picture of cloud stability.

Cloud boundedness is significantly affected by environmental tidal forces, varying across galactic regions.

Abstract

We investigate the role of tidal forces in molecular cloud formation by examining how apparent boundedness, as diagnosed by the classical virial parameter, relates to the actual gravitational state of clouds subject to tidal forces from their environment. Clouds are identified by a dendrogram algorithm in zoom-in regions taken from a simulation of a Milky Way-mass galaxy with the Voronoi mesh code AREPO that resolves star-forming regions at sub-parsec resolution. To look at a range of environments, we use data from three different regions that evolve differently in the center, near the equivalent of the Solar circle, and the outskirts of the modeled galaxy, at three different times, each spaced 2 Myr apart. We compute the importance of tidal forces on all identified clouds. We then compare the boundedness of clouds including only their internal potentials to boundedness also including the external gravitational potential. This comparison shows that tidal forces can unbind apparently bound clouds and bind apparently unbound clouds. We characterize the cloud population by comparing their virial parameters to their surface densities, finding the ratio of the maximum to the minimum eigenvalues of the tidal tensor, and determining the strength of gravitational instability in each examined region. We find that it is necessary to take the total gravitational potential into account rather than just the internal self-gravity of the clouds to have an accurate understanding of cloud dynamics.