Connecting Star Formation in the Milky Way and Nearby Galaxies. I. Comparability of Molecular Cloud Physical Properties

TL;DR

This study compares molecular cloud properties in the Milky Way and nearby galaxies, revealing that when accounting for observational resolution, their physical characteristics are broadly similar, with environmental effects influencing some clouds.

Contribution

It establishes criteria to correct for resolution effects in extragalactic observations, enabling consistent comparison of molecular cloud properties across different galaxies.

Findings

Cloud properties are comparable across galaxies after correcting for resolution effects.

Environmental factors can cause elevated velocity dispersions in some extragalactic clouds.

Clump-associated clouds are more massive, denser, and show signs of stronger feedback.

Abstract

We used CO (2-1) and CO (1-0) data cubes to identify molecular clouds and study their kinematics and dynamics in three nearby galaxies and the inner Milky Way. When observed at similar spatial and velocity resolutions, molecular clouds in the same mass range across these galaxies show broadly comparable physical properties and similar star formation rates (SFRs). However, this comparability depends on smoothing Milky Way clouds to match the resolution of the extragalactic observations. The beam effect can artificially inflate cloud sizes, leading to inaccurate estimates of radius, density, and virial parameters. By comparing high-resolution and smoothed Milky Way data, we established criteria to exclude beam-affected clouds in the extragalactic sample. After applying this filter, cloud properties remain consistent across galaxies, though some clouds in NGC 5236 show elevated velocity dispersions, likely due to environmental effects. In the inner Milky Way, molecular clouds fall into two groups: those with clumps and those without. Clump-associated clouds are more massive, denser, have higher velocity dispersions, lower virial parameters, and stronger 8~\(\mu\)m emission, suggesting more intense feedback. Strong correlations are found between cloud mass and total clump mass, clump number, and the mass of the most massive clump. These results suggest that a cloud's physical conditions regulate its internal clump properties and, in turn, its star-forming potential.