Metal-poor molecular clouds in dwarf galaxies at parsec-scale resolution

Abstract

Understanding how stars form at low metallicity is challenging because carbon monoxide (CO) emission, which usually probes the molecular gas reservoir, is faint, and tends to be concentrated in parsec-scale clumps. To overcome this difficulty, in order to study the molecular clouds in metal-poor environments, we have acquired CO(2-1) ALMA maps at 3-4 pc resolution of four nearby dwarf galaxies with nebular (O/H) metallicities ranging from roughly 1/9 Z_sun to 1/3 Z_sun. Significant CO signal in the cubes has been identified through two masks, a ``strict'', high-confidence, mask and a ``broad'' one for better completeness. The former traces the cores of the clouds, while the latter includes fainter emission in the vicinity of the cores. Through a cloud-finding algorithm applied to the two sets of masked cubes, we have identified more than 200 molecular clouds across the sample, with roughly 50 in NGC3109, at 1/9 Z_sun, the lowest metallicity parsec-scale cloud population yet observed. The best-fit size-linewidth relations for the resolved clouds are similar among all galaxies, with a slope like the Milky Way (MW), but with lower velocity dispersion at a given cloud size. For a fixed CO-to-H_2 MW conversion factor, the cloud virial ratios tend to be large, and show a weak increasing trend with increasing cloud size, and decreasing apparent surface densities and metallicity. We find evidence for faint but significant CO emission outside CO-bright cores, spanning similar velocities to those observed in the cores. We attribute this to effective shielding by gas and dust, despite the low metallicities of our sample, since the CO tends to be cospatial with dust as traced by far-infrared emission.