ALMA Observations of a Quiescent Molecular Cloud in the Large Magellanic Cloud

TL;DR

This study uses ALMA to observe a quiescent molecular cloud in the Large Magellanic Cloud, revealing that its properties differ significantly from active star-forming regions, with implications for understanding molecular cloud diversity.

Contribution

First high-resolution ALMA observations of a quiescent molecular cloud in the LMC, comparing its structure and dynamics to a star-forming cloud, highlighting non-universality of cloud properties.

Findings

Structures in the quiescent cloud have lower surface density and velocity dispersion.

Higher surface density structures are closer to virial equilibrium.

Small-scale energetics are dominated by a few structures, causing scatter in size-linewidth relations.

Abstract

We present high-resolution (sub-parsec) observations of a giant molecular cloud in the nearest star-forming galaxy, the Large Magellanic Cloud. ALMA Band 6 observations trace the bulk of the molecular gas in $^{12}$CO(2-1) and high column density regions in $^{13}$CO(2-1). Our target is a quiescent cloud (PGCC G282.98-32.40, which we refer to as the "Planck cold cloud" or PCC) in the southern outskirts of the galaxy where star-formation activity is very low and largely confined to one location. We decompose the cloud into structures using a dendrogram and apply an identical analysis to matched-resolution cubes of the 30 Doradus molecular cloud (located near intense star formation) for comparison. Structures in the PCC exhibit roughly 10 times lower surface density and 5 times lower velocity dispersion than comparably sized structures in 30 Dor, underscoring the non-universality of molecular cloud properties. In both clouds, structures with relatively higher surface density lie closer to simple virial equilibrium, whereas lower surface density structures tend to exhibit super-virial line widths. In the PCC, relatively high line widths are found in the vicinity of an infrared source whose properties are consistent with a luminous young stellar object. More generally, we find that the smallest resolved structures ("leaves") of the dendrogram span close to the full range of line widths observed across all scales. As a result, while the bulk of the kinetic energy is found on the largest scales, the small-scale energetics tend to be dominated by only a few structures, leading to substantial scatter in observed size-linewidth relationships.