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There is a group of powerful compact radio sources which have two almost equal flux density components on parsec scales. On the basis of multifrequency, multiepoch Very Long Base Interferometry (VLBI) observations, we conclude that there are two physically distinct classes among such sources. In one class the two components are probably 'minilobes' where oppositely directed jets terminate. Because of their intrinsic symmetry we believe the new name 'compact symmetric objects' best describes this class of object. We present detailed observations of the source 0108+388, showing that this source has properties consistent with this class, including a very small limit on relative internal motions of is ≤ 0.18/h^(-1) (c) (using H_0 = 100 h km s^(-1) Mp c^(-1) and q_0 = 0.5). We also detect a weak feature between the two main components which may be associated with the central engine in this source. We believe there exists another class of double source composed of core-jet sources in which a component in the jet is as bright as the component associated with the central engine. These sources are distinct from those of the first class in showing systematic differences in the properties of the two components. All these sources have small relative motions between the two main components (e.g., in 2021+614 we detect a motion of 0.13 h^(-1)(c) and in 0711+356 an upper limit of 0.4 h^(-1)(c)). The combination of large relative brightness for the secondary and the low velocities might be explained if the secondary is due to a bright standing shock in the jet. Alternatively, the bright secondary component might be due to a relativistic flow close to the line of sight at a point along a curving parsec-scale channel (as has been proposed for 4C 39.25).

Evidence for two classes of parsec-scale radio double source in active galactic nuclei