QUASARS AND ACTIVE GALACTIC NUCLEI IN RICH ENVIRONMENTS .2. THE EVOLUTION OF RADIO-LOUD QUASARS

The investigation of the galaxy cluster environments of large samples of quasars allows us to determine the effects of environment on the evolution of their nuclear activity. The galaxy cluster environments of a sample of 63 quasars with m > 17.5 and 0.3 < z < 0.6 are presented. The quasar-galaxy spatial covariance amplitude, B(gq), is used to calculate the richness of the galaxy cluster environment within 0.5 Mpc of each quasar. This quantity is calculated using self-consistent models for the galaxy luminosity and spatial distributions and for the background galaxy counts. When combined with previous results for bright quasars, these data show that radio-loud quasars in environments richer than Abell class O evolve more rapidly than those in poor environments, a difference significant at the 96% confidence level. The ensemble of radio-loud quasars in these rich environments fades approximately 4 mag between z = 0.6 and z = 0.3, corresponding to an e-folding time of 0.9 Gyr. This time scale is similar to dynamical time scales in these rich cluster cores; the evolution of the physical conditions within the cluster may therefore be responsible for this rapid evolution in quasar activity. In particular, changes in the velocity dispersion or the intracluster medium are suggested as possible mechanisms for driving the observed evolution. Unlike radio-loud quasars, radio-quiet quasars are rarely found in environments as rich as Abell class I clusters. This difference in environment, significant at the 99% confidence level, indicates that these are physically different objects. This result is also in agreement with scenarios linking radio-loud quasars with elliptical host galaxies, and radio-quiet quasars with spiral host galaxies.