KILOPARSEC-SCALE MOLECULAR GAS EXCITATION IN SPIRAL GALAXIES

We combine beam-matched (CO)-C-13, (CO)-C-12 J = 3 --> 2 and J = 2 --> 1 line data to infer the molecular gas excitation conditions in the central 500 to 1600 pc diameters of a small sample of infrared-bright external galaxies: NGC 253, IC 342, M83, Maffei 2, and NGC 6946. We find that the central 170 to 530 pc diameter regions have typical molecular gas densities ranging from less than or similar to 10(4) cm-3 (in M83) to greater than or similar to 10(5) cm-3 (in NGC 253) and that, outside of these regions, the densities are likely to be less than or similar to 10(4) cm-3. The molecular clouds outside the inner 170-530 pc are at least as warm as the molecular clouds in our Galaxy. Column densities derived from integrated (CO)-C-13 line strengths and Halpha surface brightnesses suggest that the star formation rate is enhanced in the central 170-530 pc diameters by an order of magnitude over that inferred for the outer star-forming disks in spiral galaxies. The total luminosity of each sample galaxy over all CO rotational lines is approximately 10(5)-10(6) L., which is within an order of magnitude of that of the important cooling line of [C II] at 158 mum. Additional observations of the J = 1 --> 0 lines of (CO)-O-18 and (CO)-C-13 Suggest that emission from the central kiloparsec of spiral galaxies may be optically thick in the (CO)-C-13 J = 1 --> 0 line. If so, estimates of molecular gas mass using (CO)-C-13 integrated line strengths could be too low. The sum of the molecular line observations imply the existence of two molecular gas components: a low-density [n(H-2) less than or similar to 10(3)-10(4) cm-3] component and a warm (T(K) greater than or similar to 50 K), high-density [n(H-2) greater than or similar to 10(4)-10(5) cm-3] component. The warm, dense component is probably associated with star formation.