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5.3.6 Cosmic Evolution

If radio galaxies and quasars are related strictly through aspect, then the relative numbers and luminosities of each class should be broadly comparable at all cosmological epochs. Indeed, the cosmological evolution of steep-spectrum radio quasars, flat-spectrum radio quasars, and radio galaxies can be described in each case by luminosity increasing with redshift roughly as (1 + z)3 out to z ~ 2, followed by a comparable decline in co-moving density at higher redshifts (Dunlop and Peacock 1990). This means current samples are broadly consistent with unification independent of redshift.

Over cosmic time, changes in the relative strengths of beamed and unbeamed components in individual sources, or in the opening angle of an obscuring torus (Lawrence 1991), might be expected. In either of those cases, the observed evolution would be different depending on orientation angle. The magnitude of this effect is clearly model-dependent but if the unification of radio galaxies and quasars is correct, it can not be large given the observed similarity in their evolutionary properties.