CO and neutral gas in the disk of the spiral galaxy IC 342

We present "on-the-fly," fully sampled maps of CO(1-0) in the central 15' of the spiral galaxy IC 342. In addition to the bright CO nuclear peak, there is a prominent CO 2'x5' bar and an extensive CO disk. The bar and nucleus contain 30% of the total observed CO emission in IC 342. Beyond the bar the CO disk contains two spiral arms, which coincide with the two inner optical arms. The substantial inter-arm CO component within this inner region has a mean surface density of 8 M-. pc(-2), close to the mean surface density of 10 M-. pc(-2), that extends to a radius of 7' (4 kpc). The total inferred H-2 mass is 7 x 10(8) M-. which is 30% of the total H I mass. We combine the CO data with VLA H I maps to obtain a map of the total gas surface density (Sigma (HI) + Sigma (H2)) in IC 342. The gas surface density shows a centrally peaked disk, dominated by H-2 to a radius of 5' (3 kpc). Spiral arms run continuously from the inner to outer galaxy, transitioning smoothly from predominantly molecular in the inner galaxy to predominantly atomic at large radii. On a global scale, the gas surface density is spatially correlated with optical spiral arm structure. On 1' (600 pc) scales the disk displays bar and arm asymmetries, azimuthal displacements of CO and H I emission, and structure that becomes increasingly complex with increasing galactic radii. We find an excellent correlation between 21 cm radio continuum and the total gas surface density.