Resolved measurements of X _CO in NGC 6946

We present the largest sample to date of giant molecular clouds (GMCs) in a substantial spiral galaxy other than the Milky Way. We map the distribution of molecular gas with high resolution and image fidelity within the central 5kpc of the spiral galaxy NGC 6946 in the ¹²CO (J = 1-0) transition. By combining observations from the Nobeyama Radio Observatory 45m single dish telescope and the Combined Array for Research in Millimeter Astronomy interferometer, we are able to obtain high image fidelity and accurate measurements of L _CO compared with previous purely interferometric studies. We resolve individual GMCs, measure their luminosities and virial masses, and derive X _CO - the conversion factor from CO measurements to H₂ masses - within individual clouds. On average, we find that X _CO = 1.2 × 10²⁰cm^-2 (Kkms ^-1)^-1, which is consistent within our uncertainties with previously derived Galactic values as well as the value we derive for Galactic GMCs above our mass sensitivity limit. The properties of our GMCs are largely consistent with the trends observed for molecular clouds detected in the Milky Way disk, with the exception of six clouds detected within ∼400pc of the center of NGC 6946, which exhibit larger velocity dispersions for a given size and luminosity, as has also been observed at the Galactic center.