Optical properties of spiral galaxies with strong nuclear radio sources : active nuclei, starbursts, and composite objects.

New optical spectrophotometry, accurate positions, and narrow-band emission-line images have been obtained for the nuclei of spiral and irregular galaxies containing strong radio sources. The optical data support a general division of these objects into those powered by star formation and those dominated by nonthermal processes. In several cases, however, nuclei are classified differently from radio and optical properties, suggesting a composite nature for some nuclei. Further work on this group could indicate whether the two processes are causally related and in which direction. All these nuclei show emission lines, systematically stronger than in a sample of optically selected nuclei. The emission-line spatial distributions, in most cases, are much like those in normal (optically selected) nuclei. Correlations between emission-line and 6 cm luminosities are explored for each class of nucleus. A correlation with forbidden N II luminosity for (optically) nonstellar-powered nuclei suggests a common energy source for the emission-line region and the radio emission, while uncertainties in optical reddening obscure any relation for nuclear H II regions.