High-redshift evolution of radio galaxies and quasars

The results of an extensive optical /infrared study of radio galaxies and quasars in the Parkes Selected Regions are presented. The data are then analysed to place new constraints on the high -redshift evolution of the radio -source population, and also to investigate the ages and star -formation histories of the galaxies in the sample.leting the identification programme commenced by Dowries et aI. (1986). CCD images (in both B and R) have been obtained for all the sources which Downes et al. failed to identify on sky- survey plate material. This has resulted in 71 new identifications, raising the overall identification content of the sample to 96 %. In addition, new spectroscopic observations have increased the fraction of measured redshifts in the sample to 43 %, while K photometry has been obtained for a complete subsample comprising -2/3 of the sources. The infrared data is used to provide accurate redshift estimates for the faintest galaxies in the sample, as well as to compliment the optical photometry in the study of spectral evolution.After incorporation of these new data, the Selected Regions sample is combined with the existing brighter samples at 2.7GHz to form a large, complete database which is then analysed to determine the form of the Radio Luminosity Function (RLF) at high redshift. The most important results of this study are the confirmation of the redshift cutoff in the flat- spectrum population (first discovered by Peacock 1985), and the discovery (for the first time) of a corresponding redshift cutoff for steep- spectrum sources. These results are demonstrated first using the free -form modelling technique of Peacock & Gull (1981), and subsequently by a variety of model- independent approaches. Finally it is shown that the evolution of the RLF can be described by a model of pure luminosity evolution, involving negative luminosity evolution beyond z - 2. The implications of these results are briefly discussed.An analysis of the optical (B -R) and optical -infrared (R -K) colours of the galaxies in the sample is carried out in order to investigate their ages and star -formation histories. This is done through comparison with the predictions of theoretical models of spectral evolution. All the galaxies display at least passive evolution and a large fraction exhibit more active behaviour. It is found to be difficult, with simple models, to reproduce simultaneously both the R -K and B -R evolution - the former implying older, more passive models than the latter. However, it is shown that the addition of a small quantity of UV flux is sufficient to reconcile a simple `Burst' model with both sets of colour data. Extremely old (- 18Gyr) models are required to account for the reddest galaxies, while the blueward scatter can be successfully reproduced either by assuming a range of formation redshifts, or by superimposing small starbursts on the old `Burst' model. The similarity between the colour evolution of the galaxies in the Selected Regions and that observed for the 3CR sources suggests that such evolution is not associated solely with the brightest radio luminosities.The thesis concludes with a brief investigation into the high -redshift evolution of optically -selected quasars, and a discussion of some of the possible connections between radio - source and star -formation activity