The statistics of radio galaxies & quasars at high redshift

Abstract

ABSTRACT.Evidence is now accumulating that the most powerful extragalactic radio sources display a cutoff in comoving density at relatively low redshift.For both radio galaxies and quasars, the luminosity function falls by a factor > 3 between ζ = 2 and 4. We describe the data which lead to this conclusion, and discuss the prospects for studying the behaviour of low-luminosity active galaxies at high redshift.The strong evolution of highly luminous radio-quiet quasars at ζ ) 2 may be an artifact of gravitational lensing. EVOLUTION OF POWERFUL RADIO SOURCESThis paper is concerned with recent studies of the population evolution of extragalactic radio sources. We are now approaching a point where the empirical change of the Radio Luminosity Function with epoch is known: this will provide a firm constraint on physical mechanisms for the evolution.A good review of this field is given by Wall (1983); at that time the most recent study of RLF evolution was that by Peacock & Gull (1981).Since then papers by Condon (1984) and Peacock (1985) have appeared.All these workers use model fitting to derive their conclusions: there is no other way of combining partial data such as source counts with complete redshift data from bright samples. The problem with this approach is the non-uniqueness of the resulting model RLF. Condon (1984) gives only one model: from this, it is impossible to know whether any given feature is required by the data or whether it is an artifact of the model formulation.In contrast, Peacock (1985) considers an ensemble of 5 different consistent RLFs to estimate the uncertainty allowed by the data. Where these differ, we may be sure the data are inadequate.Where they agree, this may be fortuitous, but is more likely to be a hint that some feature is required by the data: we can then construct a specific test to see if this is the case.