Summary

We have discussed how a dual-population unification scheme successfully describes the evolution and Doppler beaming of powerful extragalactic radio sources. The evolution model is well-supported by observational data and reflects the `FRII/quasar epoch' at 2 - 3 attributable to very strong cosmic evolution of the FRII population. In contrast, FRIs have a constant space density with epoch, where this is almost certainly due to the population being replaced at a rate which matches their radio active lifetime.

Whilst a physical understanding of how individual radio-loud AGN evolve is not yet available we hypothesize that luminosity evolution produces a transition from FRII to FRI-type sources from 2 - 3 to the present epoch. Even though we do not understand the nature of the physical process which caused this evolution, powerful radio sources are a reliable trace of large-scale structure, although further work is required to refine the redshift distribution of radio sources to mJy-flux density limits.

© Copyright Astronomical Society of Australia 1997