First observations of individual molecular clouds in the irregular galaxy NGC 6822

Three molecular clouds have been mapped in the nearby metal-poor dwarf irregular galaxy NGC 6822 using the Owens Valley Millimeter-Wave Interferometer. These molecular clouds are similar to molecular clouds recently studied in the Small Magellanic Cloud, and are somewhat less massive on average than molecular clouds in Local Group spiral galaxies. Based on upper limits to the virial masses of the clouds, the CO-to-H2 conversion factor in NGC 6822 is <2.2 ± 1.3 times larger than the Galactic value [3 × 10²⁰ cm^-2 (K km s^-1)^-1]. Conversion factors obtained for Galactic molecular clouds, NGC 6822, and the SMC are consistent with the variation in the CO-to-H2 conversion factor being linearly proportional to the oxygen abundance of the galaxy. However, the uncertainties are sufficiently large that equal conversion factors in NGC 6822 and the Galaxy cannot be ruled out. The gas depletion time obtained for the H II region Hubble V is comparable to those of giant H II regions in M33. Thus high-efficiency star formation is not limited to the giant H II regions seen spiral galaxies, but can also occur in a small group of relatively low-mass (a few times 10⁴ M⊙) molecular clouds.