Arm and interarm molecular clouds in M 83.

Observations of the J=1-0 and J=2-0 transitions of CO in a selected region of M 83 show a well-defined molecular cloud spiral arm segment. The molecular clouds are situated 300 pc downstream from the dust lane. Comparison of our CO observations with interferometric observations of HI and Hβ emission reveals that the location of the molecular component coincides with both the HI and the ionized gas regions. The J=2-0/J=1-0 intensity ratio is close to one for both the arm and center regions. The interarm region has an average line ratio of 1.8. We estimate that 60% of the interarm emission, corresponding to 12% of the H_2_ mass, originates from an optically thin CO component. A lower limit to the beam averaged arm-interarm mass ratio is 4.5. The optically thin emission may originate from a population of dense and warm, but very small molecular clouds.