In either case, different parts of the compact region become optically thick at different frequencies, which can flatten the integrated spectrum of the compact source over a suitably broad frequency range. A jet extending from the compact source to the extended radio lobes is observed on one side of the source. I sources are not edge-brightened, which suggests that their outflows are subsonic. Data courtesy of A. H. Bridle, figure by R. W. Pogge. An example of a jet is seen in the FR II source shown in Fig.
Jets appear to originate at the central compact source and lead out to the extended lobes.
They often show bends or wiggles between the central source and the point where the jet appears to expand into the extended radio structure. The appearance of jets suggests that they transport energy and particles from the compact source to the extended regions. The relative strength of the extended, compact, and jet components varies with frequency since the different components have different spectral shapes. This will be discussed further in Chapters 4 and 7. Because of the different observing frequencies and detection limits of the various radio surveys used to find quasars, each of the major radio-source catalogs contains certain selection biases.