Compact and High Excitation Molecular Clumps in the Extended Ultraviolet Disk of M83

TL;DR

This study investigates the structure of molecular clouds in the XUV disk of M83, revealing compact dense clumps similar in size to local star-forming regions, which are key to understanding star formation in low-metallicity environments.

Contribution

The paper provides high-resolution ALMA observations confirming the presence of compact dense clumps in XUV disks, supporting a universal cloud structure for star formation across different galactic environments.

Findings

Dense clumps have diameters below 6-9 pc, similar to Orion A.

Dense star-forming clumps are common in both normal and XUV disks.

Cloud structure influences star formation more than external triggers.

Abstract

The extended ultraviolet (XUV) disks of nearby galaxies show ongoing massive star formation, but their parental molecular clouds remain mostly undetected despite searches in CO(1-0) and CO(2-1). The recent detection of 23 clouds in the higher excitation transition CO(3-2) within the XUV disk of M83 requires an explanation. We test the hypothesis: the clouds in XUV disks have a clump-envelope structure similar to those in Galactic star-forming clouds, having star-forming dense clumps (or concentrations of multiple clumps) at their centers, which predominantly contribute to the CO(3-2) emission, surrounded by less-dense envelopes, where CO molecules are photo-dissociated due to the low-metallicity environment there. We utilize new high-resolution ALMA CO(3-2) observations of a subset (11) of the 23 clouds in the XUV disk. We confirm the compactness of the CO(3-2)-emitting dense clumps (or their concentrations), finding clump diameters below the spatial resolution of 6-9~pc. This is similar to the size of the dense gas region in the Orion A molecular cloud, the local star-forming cloud with massive star formation. The dense star-forming clumps are common between normal and XUV disks. This may also indicate that once the cloud structure is set, the process of star formation is governed by the cloud internal physics rather than by external triggers. This simple model explains the current observations of the clouds with ongoing massive star formation, although it may require some adjustment, e.g., including an effect of cloud evolution, for a general scenario of star formation in molecular clouds.