The structure of the inner eastern spiral arm of M83

The spatial distributions of dust, Hβ, and nonthermal radio emission are studied in the inner eastern arm of M83 (NGC 5236) using VLA 20 cm continuum radio observations, a pure Hβ image, and a red continuum CCD image. Contrary to the case in, e.g., M51 and M101, the radio continuum emission in the neighborhood of the giant H II complexes is dominantly nonthermal. Nonthermal radio emission is also found to correspond well with the dust lanes, especially in the bar near the nucleus where the radio continuum is bright and the dust lanes are nearly perpendicular to the galaxy rotation. However, farther out along the eastern spiral arm, this "dust-lane" component of the nonthermal radio continuum fades, becoming overshadowed by a parallel ridge of strong nonthermal emission located well outside the dust lanes and following the general locus of the H II regions. We propose that this emission is mainly due to an enhanced density of cosmic rays from relatively recent supernovae, of which M83 is an unusually copious producer. Such an increase in the local cosmic ray density is also likely to be responsible for the enhanced CO emission in this region of the arm.