CO(1-0), CO(2-1), and neutral gas in NGC 6946: Molecular gas in a late-type, gas-rich, spiral galaxy

We present on-the-fly maps of the CO(1-0) and CO(2-1) emission covering a 100; 100 region of NGC 6946. Using our CO maps and archival VLA H I observations we create a total gas surface density map, Sigma 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: 50 ( 6 kpc). We estimate that the total H-2 mass is 3; 109 M-circle dot, 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 widespread optically thin gas. Star formation tracers are better correlated with the total neutral gas disk than with the molecular gas by itself, implying Sigma(SFR) / Sigma(gas). Using the 100 mu m 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.