Physical evolution: How do the sources evolve ?

Our analysis has adopted a simple parametric form for the evolution of the source populations. However, whilst a parametric approach yields a model comprising only a limited number of free parameters, we must be careful interpreting the overall results, as it is subject to the following limitations:

1.

It cannot determine the type of evolution a population has undergone. The successful evolution model requires that the FRII population undergoes density evolution with the magnitude of the evolution being luminosity-dependent. However, due to the shape of the radio luminosity function this evolution mimics pure luminosity evolution. Thus our model cannot discriminate between the different evolutionary scenarios of luminosity or density evolution or a combination of both.

2.

It tells us nothing about the physical evolution of individual sources. The model fits the behaviour of each population so that our evolution model tells us nothing about the physical evolution of a single source. All we can conclude is that FRIIs undergo significant cosmic evolution whilst the FRIs have a constant space density with epoch. Likewise the derived beaming parameters cannot be applied to individual objects, instead they represent some `average' values.

Moreover, a fundamental question remains unanswered - what has happened to the powerful radio sources from 2 - 3 to the present epoch ? From the form of the evolving radio luminosity function (RLF) we hypothesize that FRIIs have evolved to FRIs. This hypothesis rests on the result that the space density of FRIIs never exceeds that of the FRI population at any epoch. In this scenario FRIs are `exhausted' FRIIs, with powerful FRIIs peaking in both space density and luminosity at 2 - 3 and then declining towards the present epoch. In this scenario the `quasar' (i.e. `FRII') epoch coincides with, and is linked to, galaxy formation: powerful radio sources form from largest mass fluctuations in the primaeval universe and evolve rapidly due to their central massive black holes (Silk & Rees 1998). Associated with this rapid evolution is the possibility of galactic mergers which would have further boosted the fuel supply to the AGN. The end result being that sources which were very luminous at 2 have now exhausted their fuel.

© Copyright Astronomical Society of Australia 1997