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

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 (0". 9 approximate to 2 pc) spatial resolution while covering a large area: four 250 pc x 250 pc regions that encompass similar to 2/3 of NGC. 6822's star formation. In these regions, we resolve similar to 150 compact CO clumps that have small radii (similar to 2-3 pc), narrow line width (similar to 1 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 15 x larger sample. At parsec scales, CO emission correlates with 8 mu m emission better than with 24 mu m emission and anticorrelates with H alpha, 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 similar to 1-2 x 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 similar to 20-25 x 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 similar to 3-5 x 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.