The Infrared-to-Radio Ratio within NGC 5236 (M83) and NGC 6946

March 1989 • 1989ApJ...338L..53B

Authors • Bicay, M. D. • Helou, G. • Condon, J. J.

Abstract • A detailed comparison of the distribution of 60 micron infrared and 20 cm radio continuum emission within NGC 5236 (M83) and NGC 6946 is presented. Local maxima in the thermal infrared and nonthermal radio maps are found to be spatially coincident on scales <= 0.3h^-1^ kpc. Superposed on this broad correlation, we observe in the disks of both galaxies a slow decrease of the 60 micron-to-20 cm ratio (Q_60_) with increasing radius. Values of Q_60_ within the central regions are enhanced by roughly a factor of 3 with respect to the outer disks, whereas the corresponding enhancement in radio surface brightness is at least a factor of 20. The 100 micron-to-20 cm ratio shows a similar but shallower gradient. To account for these observations, we propose that spiral galaxies are characterized by an infrared disk with a shorter scale length than that of the radio continuum disk, the latter being smeared as a result of cosmic-ray propagation.

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