A study of 3CR radio galaxies from z=0.15 to z=0.65.: I.: Evidence for an evolutionary relationship between quasars and radio galaxies

Deep optical images have been gathered for a nearly complete sample of radio galaxies from the Revised Third Cambridge (3CR) Catalog in the redshift range 0.15 < z < 0.65. Total and nuclear magnitudes and colors have been extracted. The richness of the galaxy clustering environment has also been quantified by calculating the amplitude of the galaxy-galaxy spatial covariance function showing (B-gg), overdensities ranging up to Abell class 0-1 clusters. These optical data are compared with similar data from an existing sample of radio-loud quasars in the same redshift range for the purpose of determining the relationship between radio galaxies and quasars. In the range 0.15 < z < 0.4, we find that quasars and radio galaxies have significantly different environments in that only radio galaxies are found in rich cluster environments. This comparison appears to rule out the hypothesis that all quasars are radio galaxies viewed from a particular angle at the 97% confidence level (99.6% confidence level if narrow-line galaxies are considered quasars). The existence of quasars in clusters at z > 0.4 supports the hypothesis that some radio-loud quasars may dim with time and evolve into radio galaxies with an e-folding time of similar to0.9 Gyr. A compatible scenario is presented for this evolution in which the quasar dims because of the absence of low-velocity interactions between the quasar host and companion galaxies, which trigger quasar activity, and/or a diminishing fuel supply caused by the more effective gas "sweeping" of a growing intracluster medium.