Kiloparsec-scale molecular gas excitation in spiral galaxies

Abstract

We combine beam-matched ^13^CO, ^12^CO 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 <~ 10^4^ cm^-3^ (in M83) to >~ 10^5^ cm^-3^ (in NGC 253) and that, outside of these regions, the densities are likely to be <~ 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 ^13^CO line strengths and Hα 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 ~10^5^-10^6^ L_sun_, which is within an order of magnitude of that of the important cooling line of [C II] at 158 microns. Additional observations of the J = 1 - 0 lines of C^18^O and ^13^CO suggest that emission from the central kiloparsec of spiral galaxies may be optically thick in the ^13^CO J = 1 - 0 line. If so, estimates of molecular gas mass using ^13^CO 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_) <~ 10^3^-10^4^ cm^-3^] component and a warm (T_K_ >~ 50 K), high-density (n(H_2_) >~ 10^4^-10^5^ cm^-3^] component. The warm, dense component is probably associated with star formation.