Physical Properties of Molecular Clouds at 2 pc Resolution in the Low-metallicity Dwarf Galaxy NGC 6822 and the Milky Way

Abstract

We present the Atacama Large Millimeter/submillimeter Array survey of CO(2–1) emission from the 1/5 solar metallicity, Local Group dwarf galaxy NGC 6822. We achieve high ( pc) spatial resolution while covering a large area: four 250 pc × 250 pc regions that encompass of NGC 6822's star formation. In these regions, we resolve compact CO clumps that have small radii (∼2–3 pc), narrow line width ( km s −1 ), and low filling factor across the galaxy. This is consistent with other recent studies of low-metallicity galaxies, but here shown with a larger sample. At parsec scales, CO emission correlates with emission better than with emission and anticorrelates with H α , so that polycyclic aromatic hydrocarbon emission may be an effective tracer of molecular gas at low metallicity. The properties of the CO clumps resemble those of similar-size structures in Galactic clouds except of slightly lower surface brightness and with CO-to-H 2 ratio ∼1–2× the Galactic value. The clumps exist inside larger atomic–molecular complexes with masses typical for giant molecular clouds. Using dust to trace H 2 for the entire complex, we find the CO-to-H 2 ratio to be the Galactic value, but with strong dependence on spatial scale and variations between complexes that may track their evolutionary state. The H 2 -to-H i ratio is low globally and only mildly above unity within the complexes. The ratio of star formation rate to H 2 is higher in the complexes than in massive disk galaxies, but after accounting for the bias from targeting star-forming regions, we conclude that the global molecular gas depletion time may be as long as in massive disk galaxies.