Weak nuclei of powerful radio sources - spectra and polarizations

Observations are presented of the spectra and polarizations of the weak cores of extended extragalactic radio sources. These cores typically show peaked spectra, and, below a luminosity of about 10 to the 26th W/Hz, are unpolarized with 90 percent confidence upper limits of about 0.6 percent. In both of these properties the cores distinguish themselves as a class from strong, active compact sources, such as those that show superluminal motion. The relation between these weak cores and other classes of compact sources and the implications for unified models based on relativistic beaming are briefly discussed. It is concluded that there is a distinct class of compact objects, different from relativistic jets, which appear in the form of separate weak or dominant cores, or as the base component in superluminal jets. 78 references.