Relativistic jets as compact radio sources

It is suggested that the variable radio emission from active galactic nuclei and quasars originates within a collimated relativistic jet of the type that has been inferred to supply extended radio sources with mass, momentum, and energy and is directly observed in several cases. An idealized description of the steady radio emission from a relativistic jet is presented, the variable component of the emission is associated with shock waves traveling in the jet, and the dynamical and radiative properties of accelerated clouds and of velocity disturbances that steepen to form propagating shocks are examined. It is shown that several observed features of compact radio sources can be interpreted on the assumption that these sources are relativistic jets viewed at small angles to their axes. Some general observational tests are proposed.