In this project we try to follow the evolution of the Radio Luminosity Function (RLF) with redshift by observing a set of galaxy clusters with known spectroscopic redshifts. This method has the advantage over previous studies, based on general radio surveys, that most of the redshifts are accurately known, that intrinsically weaker sources are well represented, and that the parent sample of galaxies is well defined. The chief disadvantages are that, because the sample is optically selected, there are few high luminosity radio sources, and that we cannot examine differences between cluster and field populations.

In this paper we present the first results of the Westerbork-Berkeley Deep Survey, the purpose of which is to derive the epoch dependence of the radio luminosity function (RLF) and the optical spectral energy distribution (SED) of elliptical radio galaxies. From calibrated photographic photometry we conclude that no spectral evolution is seen for z ≲ 0.4, but that colors of radio galaxies are bluer for 0.4 ≲ z ≲ 0.9 than predicted from classical model spectra for ellipticals (with star formation only during the first 109 yrs after formation). First-ranked cluster radio galaxies may be slightly bluer than the average radio galaxy for z ≳ 0.6. The epoch dependence of the 1.4 GHz RLF is determined in a direct way. We find almost no population evolution for z ≲ 0.3, but strong evolution for 0.3 ≲ z ≲ 0.9 for all radio powers above the break in the RLF, with enhancement factors of ∼ 100 at z ∼ 0.8.

Recent theoretical models of radio radiation from central cores of radio sources invoke axial relativistic beaming of the radiation and predict significant enhancement of the radiation from the central cores when viewed at small angles to the direction of the beam. Investigations of the angular diameter distributions of radio sources by Swarup (1975) has shown that the median angular size of radio sources below 4 Jy at 327 MHz is about 10 arc seconds. Hence a sample of sources, with angular sizes substantially less than the above median, chosen from the Ooty occultation survey may be expected to contain a large fraction of double sources viewed along the axis of the double structure.

The purpose of this communication is to update our knowledge of the radio structural properties of quasars and blank field radio sources (blank field ≡ any radio source without an identification on the Palomar Sky Survey prints). The quasar sample consists of all sources (25) with angular sizes greater than 10 arcsec in the list of Jodrell Bank quasars observed by Owen, Porcas and Neff (1978). The blank fields consist of 16 3CR sources also with structures >10 arcsec based on Cambridge 5 km telescope observations. The sources were selected in low-frequency surveys; their emission at ν < 1 GHz is dominated by extended components with steep spectra. Thus, both samples should be oriented randomly in space except for a slight bias to be in the plane of the sky.

Even though the number of known quasars is approaching 2000, relatively few belong to well defined complete samples which are needed to derive their space distribution. For many years, the Braccesi survey (Braccesi, Formiggini, and Gandolfi 1970) was the only published one — its limited spectroscopic coverage allowed construction of only a small complete sample of 19 quasars brighter than B = 18, over an area of 36 sq. deg.

We describe recent advances in observation and analysis which lead to improved understanding of the spatial distribution of QSOs and radio galaxies.

Empirical descriptions of cosmological evolution rely on an assumed relation between luminosity distance and redshift in order to derive the luminosity function and its epoch dependence. This relation is usually taken to be that of an idealised Friedmann cosmology, despite the obvious abundance of small-scale structure in the Universe. However, the assumption that the probability of a lensing event is negligible is now made less tenable with the discovery of the double and triple QSO's, both of which appear to have been magnified by factors of 10–15. With at least two events of this amplification in 1500 known quasars, it may be that a large fraction of observed QSO's have been magnified by significant amounts. Turner (1980) suggests that all evolutionary statistics (e.g. V/Vmax) may thus be misleading, although he considers only an illustrative model of the effect. The problem is that, even if the intrinsic probability of a lensing event is very low, the effect becomes important if the background density of faint sources rises sufficiently quickly. We must therefore calculate not only the intrinsic probability of a given amplification, but incorporate this with the luminosity function to see if lensing could be made dominant by selection effects.

The density of intergalactic gas may be an important parameter in the formation of extended radio sources. It may range from ∼ 0.1 particle cm−3 in the centres of some rich clusters of galaxies down to 10−8cm−3 or less in intercluster space. The possible influence of the intracluster gas surrounding NGC 1275 on its radio emission is discussed, and the possibility that a significant fraction of the X-ray background is due to a hot intergalactic medium is explored in some detail.

The discovery of the double quasar (Walsh ET AL. 1979) provides an opportunity to study the mass distribution of elliptical galaxies and clusters of galaxies. This has been done initially by Young ET AL. (1981) who produced a model to account for the image positions and intensities. Since then VLBI observations have been made of 0957+561A and B (Porcas ET AL. 1981) which show very similar core and jet structures in the nuclei of both images. In addition to providing further evidence in favour of the gravitational lens hypothesis, these new observations provide additional constraints on the mass distribution of the lensing galaxy and cluster. We have attempted to produce a model in the light of these new results.

An intervening galaxy acting as a gravitational lens produces usually from a compact radio source two or four enlarged “crescent” shaped virtual images, which have the same surface brightness as the object, but look brighter, due to the lens caused enlargement of the object area in the image. Velocities between elements of a source being vector quantities, will also be seen enlarged, occasionally to superluminal velocities (Fig.1). It is as simple as that. (Barnothy and Barnothy 1971; Barnothy 1976.)

One of the most striking properties of the radio galaxies is the predominance of a symmetrical double lobed structure. Any model to explain the energy release and collimation in radio galaxies must be able to produce this symmetry as the normal morphology. Although this large scale symmetry in radio galaxies is a well known phenomenon, I feel it is worth reemphasising since our current attention is more sharply focussed on the small scale and often much less symmetric structures which are seen with the higher resolution radio telescopes (VLB, VLA etc).

It is concluded that many, if not most, jets are truly one-sided. The hypothesis that the powerful radio emission of quasars and radio galaxies is caused by ejections of “plasmoids” originating in super-critical accretion on massive black holes is discussed. Because of asymmetry in the ejection of plasmoids from the thick accretion disks which form around massive black holes, the latter acquire considerable recoil momentum and should escape from the nuclei of the galaxies with large velocities. This provides a possibility for explaining a number of evolutionary effects and an approach to solving the problem of “dead” quasars.