Resolved Measurements of Xco 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 5 kpc
of the spiral galaxy NGC 6946 in the 12CO (J=1-0) transition. By combining
observations from the Nobeyama Radio Observatory 45-meter single dish telescope
and the Combined Array for Research in Millimeter Astronomy (CARMA)
interferometer, we are able to obtain high image fidelity and accurate
measurements of LCO compared with previous purely interferometric studies. We
resolve individual giant molecular clouds (GMCs), measure their luminosities
and virial masses, and derive Xco - the conversion factor from CO measurements
to H2 masses - within individual clouds. On average, we find that Xco = 1.2
\times 10^20 cm-2 / (K km s-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 \sim400 pc
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.
