Angular Size-Redshift Test for Compact Radio Sources in the Caltech-Jodrell Bank Surveys

In 1958 Hoyle first suggested that the angular separation of the components of large double radio sources might be used as a standard rod in the classical angular diameter vs. redshift (θ-z) test (Hoyle 1959). Thirty years and thousands of radio observations later this hope was finally dashed. Gradually it had become clear that the evolution of the intrinsic sizes of double radio sources with cosmic epoch completely masked the more subtle effects of universal geometry (e.g. Nilsson et al. 1993). It was therefore with some surprise that the community greeted the analyses of the angular sizes of the milliarcsecond jets in bright radio nuclei by Kellermann (1993) and Gurvits (1994). Rather than the monotonic fall-off of median angular size with redshift exhibited by double radio sources, the compact radio sources seemed to show the expected signature of standard relativistic cosmology i.e. an initial fall-off of θ with z followed by an approximately constant θ beyond z ∼ 0.5. Perhaps, as suggested by Kellermann (1993), the power and short lifetimes of these structures make them less susceptible to the environmental effects which bedevilled the test with double sources.