A Molecular Gas Ridge Offset from the Dust Lane in a Spiral Arm of M83

We present a high-resolution (5" x 12") interferometric map of the CO (J = 1 - 0) emission on the eastern spiral arm of M83. The detected emission originates in about five unresolved components located parallel but ~300 pc downstream from the dust lane which lies along the inner edge of the spiral arm. With one exception, the CO components in our map are all located within 130 pc of an H II region, and may represent emission from locally heated gas. While heating by H II regions may enhance the CO luminosity in the arm, the lack of CO emission on the dust lane indicates that the dense molecular gas does not pile up here in M83. The distribution of molecular emission in this spiral arm of M83 is shown to be remarkably different from the spiral arms of M51, where CO emission peaks on the dust lane. The difference between the molecular gas distributions in the two galaxies is attributed to the difference in the strength of their density waves. The observations of M83 are consistent with the model of Elmegreen, in which diffuse gas is compressed at the shock front producing the dust lane at the inner edge of the spiral arm, while dense giant molecular clouds pass through the front and form a broad distribution on the arm. In M51, with its significantly stronger density wave, both diffuse and dense gas are concentrated in a collision front located at the inner edge of the spiral arm, coincident with the dust lane.