SPATIAL-DISTRIBUTION OF FAR-INFRARED EMISSION IN SPIRAL GALAXIES .1. RELATION WITH RADIO-CONTINUUM EMISSION

We use high resolution IRAS and 20 cm radio continuum (RC) images of a sample of 22 spiral galaxies to study the correlation between the far -infrared (FIR) and RC emissions within the galactic disks. a combinat ion of exponential and Gaussian profiles rather than a single exponent ial profile is found to be a better representation of the observed int ensity profiles in the two bands. The Gaussian component, which we sho w is not due to the effects of limited beam-resolution, contains more than 60% of the total flux in majority of the galaxies. The dominance of the Gaussian component suggests that the nuclear star forming regio ns and the bulge stars are more important contributors to the emission in the two bands, rather than the outer exponential stellar disks. Th e RC profile is flatter compared to the FIR profile, resulting in a de crease of their ratio, Q(60), away from the center. However, the Q(60) increases in the extreme outer parts, where the dispersion in the FIR and RC correlation is also higher than in the central regions. The gl obal Q(60) and its dispersion match those in the inner parts of the ga laxies. These results imply that the observed tight correlation in the global quantities reflects processes in the inner regions only where OB stars and the associated Type II supernovae control the FIR and RC emission. In the outer parts heating of very small dust grains by the old disk stars provides a secondary component in the FIR emission, wit hout associated RC emission. The edge-on galaxy NGC 3079 shows extende d FLR and RC emissions along its minor axis, probably associated with the nuclear starburst activity. (C) 1997 American Astronomical Society .