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
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.