The environments of radio-loud quasars

We have obtained multi-colour imaging of a representative, statistically complete sample of low-frequency selected (S-408MHz > 0.95Jy) radio loud quasars at intermediate (0.6 < z < 1.1) redshifts. These sources are found in a variety of environments, from the field through to rich clusters. We show that statistical measures of environmental richness, based upon single-band observations are inadequate at these redshifts for a variety of reasons. Environmental richness seems correlated with the size and morphology of the radio source, as expected if the energy density in the radio lobes is approximately the equipartition value and the lobes are in pressure equilibrium with a surrounding intragroup/cluster medium. Selecting on radio size therefore efficiently selects dense galactic sytems at these redshifts.