8.2.2 Cosmic Evolution of Radio-Loud AGN
Cosmic evolution could be an important aspect of unified schemes. Realistically, one should probably expect evolution in the Lorentz factor (), in the ratio of beamed to unbeamed flux (R), and in the intrinsic fraction of luminosity radiated by the jet (f). But evolution has been largely ignored in unified schemes (cf. Vagnetti and Spera 1994) because of the poor statistics when existing samples are divided into multiple redshift bands.
In most cases, one finesses the evolution of AGN by parameterizing it with a smooth function and extrapolating evolving properties to zero redshift. For example, comparisons between beamed and observed LFs were done assuming exponential luminosity evolution (Sec. 6). The preferred approach, statistics permitting, is to derive the LFs at different redshifts and apply the beaming formalism to the single-epoch LFs. This would allow, for example, the study of evolutionary trends of the beaming parameters.
The well-identified radio-selected samples currently available - the 3CR, the 1 Jy, the 2 Jy samples - are quite shallow. For a typical radio-loud AGN spectrum, optical and X-ray surveys are much deeper, in many cases the equivalent of 1 mJy or fainter at 5 GHz. Deep radio surveys, optically identified and classified morphologically in the radio, would greatly improve our understanding of unified schemes but represent a substantial technical challenge.
Another important aspect is the evolution of individual sources and the potential consequences for their radio morphologies. For example, the observed correlation between core-to-extended radio flux and linear size (e.g., Lister et al. 1994b) can be explained by a model wherein the radio source starts out as a luminous core that fades with time as the lobes brighten (Hutchings et al. 1988). If this evolutionary effect dominates over beaming and orientation, it could complicate considerably the use of this flux ratio as a beaming indicator.