THE STATISTICS AND AGES OF COMPACT SYMMETRICAL OBJECTS

Two dual frequency VLBI surveys of the compact structures in complete high-frequency samples of radio sources have been carried out. A class ification of powerful extragalactic radio sources based on these surve ys has revealed that similar to 10% of the objects are ''compact symme tric objects'' (CSOs) of an overall size less than or similar to 1 kpc , with high-luminosity lobes straddling the center of activity. In man y respects these objects mimic Cygnus A, but are smaller by a factor o f 10(2)-10(4). The working surfaces, or ''hot spots,'' of the jets in CSOs are much closer to the central engine than in other powerful radi o sources, and they provide both a new laboratory for the study of the physics of relativistic jets and a unique probe of the interstellar m edium in the range 1-500 pc from the central engines of active galaxie s. The properties and statistics of CSOs are presented. The radio gala xy 2352+495 is shown to be a typical member of this class, and its opt ical and radio properties are discussed in detail. Our analysis of the hot spots in this object, in 0108+388, and in 0710$-439 shows that if these are confined by ram pressure then CSOs have ages much less than 10(6) yr. Two models for CSOs are investigated-a ''fast'' model in wh ich the hot spots are advancing at similar to 0.02c and a ''slow'' mod el in which the velocity of advance is similar to 4 x 10(-3)c. On the ''slow'' model, the morphology of these objects would be more spherica l than is observed, and there would likely be more reddening of the na rrow emission lines relative to the galactic continuum emission, where as all of the observations are consistent with the fast model, in whic h the speed of advance is comparable to the speed of advance of the lo bes in Cygnus A and the ages of CSOs are in the range 10(3)-3 x 10(4) yr. This has interesting implications for their evolution which are di scussed in a separate paper.