On the scale-height of the molecular gas disc in Milky Way-like galaxies

TL;DR

This study uses simulations to analyze the molecular gas disc's scale-height in Milky Way-like galaxies, revealing that accurate measurements require resolving individual molecular clouds due to the influence of local gravitational effects.

Contribution

It demonstrates that the molecular gas disc's scale-height is approximately 50 pc and highlights the importance of using cloud-scale velocity dispersion for accurate hydrostatic predictions.

Findings

The molecular gas disc has a Gaussian vertical distribution with a ~50 pc scale-height.

Hydrostatic predictions using total turbulent velocity dispersion overestimate the scale-height.

Using cloud-cloud velocity dispersion yields more accurate estimates of the molecular disc scale-height.

Abstract

We study the relationship between the scale-height of the molecular gas disc and the turbulent velocity dispersion of the molecular interstellar medium within a simulation of a Milky Way-like galaxy in the moving-mesh code Arepo. We find that the vertical distribution of molecular gas can be described by a Gaussian function with a uniform scale-height of ~50 pc. We investigate whether this scale-height is consistent with a state of hydrostatic balance between gravity and turbulent pressure. We find that the hydrostatic prediction using the total turbulent velocity dispersion (as one would measure from kpc-scale observations) gives an over-estimate of the true molecular disc scale-height. The hydrostatic prediction using the velocity dispersion between the centroids of discrete giant molecular clouds (cloud-cloud velocity dispersion) leads to more-accurate estimates. The velocity dispersion internal to molecular clouds is elevated by the locally-enhanced gravitational field. Our results suggest that observations of molecular gas need to reach the scale of individual molecular clouds in order to accurately determine the molecular disc scale-height.