The radio spectra of galactic nuclei

We present a model for the inverted radio spectra of active active galactic nuclei as well as the central regions of normal galaxies. The model is based on the unified scenario for active galaxies, stating that the central engines of active galaxies consists of a supermassive black hole surrounded by an accretion disk and a radio jet. The nuclei of normal (i.e. less active) galaxies are supposed to be scaled-down versions of the same phenomenon. We show that the radio emission of a jet component, becoming optically thin to the radio emission of a monoenergetic pair plasma at decreasing frequencies as itmoves outward and expands, is well suited to explain the observational results. We present a model calculation for the special case of the nucleus of M 81.