Models of the cosmological evolution of extragalactic radio sources – I. The 408-MHz source count

Abstract

A simple numerical procedure for analysing the counts of extra-galactic radio sources is described. The scheme makes efficient use of the extensive data base which is now available for studying the spatial distribution of radio sources. The method is first applied to data at 408 MHz. We show that, if proper statistical testing is incorporated, the technique provides strong support for the qualitative conclusions of previous analyses: the comoving density of the more powerful radio sources with extended structure is now less than at earlier epochs by a factor of ≳ 103. However, several evolution models proposed earlier are quantitatively unsatisfactory, and new models are derived which adequately describe the current data. These new models make contrasting predictions about redshift distributions amongst the faint radio sources (S408 ≽ 10 mJy), and identification data at such levels can therefore provide powerful constraints upon the models. Our results are compared with results from recent investigations of the spatial distribution of steep-spectrum radio sources.