This thesis presents extensive follow -up observations of the Combined EIS -NVSS
survey Of Radio Sources (CENSORS) with the primary scientific goal of studying
the high redshift behaviour of the radio luminosity function.
Understanding the behaviour of radio galaxies and quasars at these high red - shifts is of far -reaching importance. Massive black holes reside in all present day
galaxies with mass roughly proportional to the mass of the bulge of the galaxy.
This suggests that black hole and spheroid formation are intimately linked. Radio
loud emission from active galaxies only occurs in association with the most massive
black holes. So by studying the cosmic evolution of radio sources we are also
studying the cosmic evolution of the upper end of the black hole mass function
which has relevance to the formation of massive galaxies at high redshift.
Previous studies of radio galaxies at redshifts greater than 2 have failed to agree
on the presence or absence of a cut -off in comoving number density. CENSORS is a 1.4GHz survey compete to a flux limit of 7.2mJy in which sources were selected
on the basis of their flux and coincidence with the ESO Imaging Survey Wide
Patch D (a 3 by 2 degree field at RA: 09 51 00 and DEC: 21 00 00). Initial identification
of host galaxies was performed by comparison of the optical EIS imaging
and the radio observations from the 1.4GHz follow up observations with the Very
Large Array. Following this preparatory work, this thesis presents K -band imaging
of those host galaxies not detected (or only marginally detected) in I -band EIS
images, and optical spectroscopy which brings the sample to 60% spectroscopic
completeness. For those sources for which a spectroscopic redshift could not be
identified, photometric redshift estimates are made.
A model is constructed to investigate the variation of the radio luminosity function
with redshift. A 140 parameter grid in radio luminosity and redshift is fitted to
the redshift distribution of CENSORS and other radio surveys; the 1.4GHz source
counts and the local radio luminosity function. Evidence for a turn-over in number
density of the most luminous sources is found at z ~ 3.