2007AJ....134.1827C

We present on-the-fly maps of the CO(1-0) and CO(2-1) emission covering a 10'x10' region of NGC 6946. Using our CO maps and archival VLA H I observations we create a total gas surface density map, Σ_gas, for NGC 6946. The predominantly molecular inner gas disk transitions smoothly into an atomic outer gas disk, with equivalent atomic and molecular gas surface densities at R=3.5' (6 kpc). We estimate that the total H₂ mass is 3x10⁹ M_☉, roughly one-third of the interstellar hydrogen gas mass and about 2% of the dynamical mass of the galaxy at our assumed distance of 6 Mpc. The value of the CO(2-1)/CO(1-0) line ratio ranges from 0.35 to 2; 50% of the map is covered by very high ratio gas (>1). The very high ratios are predominantly from interarm regions and appear to indicate the presence of wide-spread optically thin gas. Star formation tracers are better correlated with the total neutral gas disk than with the molecular gas by itself, implying Σ_SFR∝Σ_gas. Using the 100 µm and 21 cm continua from NGC 6946 as star formation tracers, we arrive at a gas consumption timescale of 2.8 Gyr, which is relatively uniform across the disk. The high star formation rate at the nucleus appears to be due to a large accumulation of molecular gas rather than a large increase in the star formation efficiency. The midplane gas pressure in the outer (R>10 kpc) H I arms of NGC 6946 is close to the value at the radial limit (10 kpc) of our observed CO disk. If the midplane gas pressure is a factor for the formation of molecular clouds, these outer H I gas arms should contain molecular gas, which we do not see because this gas is beyond our detection limit.