EVIDENCE FOR 2 CLASSES OF PARSEC-SCALE RADIO DOUBLE SOURCE IN ACTIVE GALACTIC NUCLEI

There is a group of powerful compact radio sources which have two almo st equal flux density components on parsec scales. On the basis of mul tifrequency, multiepoch VLBI observations, we conclude that there are two physically distinct classes among such sources. In one class the t wo components are probably ''minilobes'' where oppositely directed jet s terminate. Because of their intrinsic symmetry we believe the new na me ''compact symmetric objects'' best describes this class of object. We present detailed observations of the source 0108 + 388, showing tha t this source has properties consistent with this class, including a v ery small limit on relative internal motions of less than or equal to 0.18 h(-1) c (using H-0 = 100 h km s(-1) Mpc(-1) and q(0) = 0.5). We a lso detect a weak feature between the two main components which may be associated with the central engine in this source. We believe there e xists another class of double source composed of core-jet sources in w hich a component in the jet is as bright as the component associated w ith the central engine. These sources are distinct from those of the f irst 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 0.1 3 h(-1) c and in 0711 + 356 an upper limit of 0.4 h(-1) c). The combin ation of large relative brightness for the secondary and the low veloc ities 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 al ong a curving parsec-scale channel (as has been proposed for 4C 39.25) .