THE STRUCTURE OF THE INNER EASTERN SPIRAL ARM OF M83

The spatial distributions of dust, Hbeta, and nonthermal radio emissio n are studied in the inner eastern arm of M83 (NGC 5236) using VLA 20 cm continuum radio observations, a pure Hbeta image, and a red continu um CCD image. Contrary to the case in, e.g., M51 and M101, the radio c ontinuum emission in the neighborhood of the giant H II complexes is d ominantly nonthermal. Nonthermal radio emission is also found to corre spond well with the dust lanes, especially in the bar near the nucleus where the radio continuum is bright and the dust lanes are nearly per pendicular to the galaxy rotation. However, farther out along the east ern spiral arm, this ''dust-lane'' component of the nonthermal radio c ontinuum fades, becoming overshadowed by a parallel ridge of strong no nthermal emission located well outside the dust lanes and following th e general locus of the H II regions. We propose that this emission is mainly due to an enhanced density of cosmic rays from relatively recen t supernovae, of which M83 is an unusually copious producer. Such an i ncrease in the local cosmic ray density is also likely to be responsib le for the enhanced CO emission in this region of the arm.