On the redshift cut-off for steep-spectrum radio sources

We use three samples (3CRR, 6CE and 6C*) selected at low radio frequency to constrain the cosmic evolution in the radio luminosity function (RLF) for the 'most luminous' steep-spectrum radio sources. Though intrinsically rare , such sources give the largest possible baseline in redshift for the compl ete flux-density-limited samples currently available. Using parametric mode ls to describe the RLF, incorporating distributions in radio spectral shape and linear size, as well as the usual luminosity and redshift, we find tha t the data are consistent with a constant comoving space density between z similar to 2.5 and z similar to 4.5. We find that this model is favoured ov er a model with similar evolutionary behaviour to that of optically selecte d quasars (i.e., a roughly Gaussian distribution in redshift) with probabil ity ratios of similar to 25:1 and similar to 100:1 for spatially flat cosmo logies with Omega (Lambda) = 0 and Omega (Lambda) = 0.7 respectively. Withi n the uncertainties, this evolutionary behaviour may be reconciled with the shallow decline preferred for the comoving space density of flat-spectrum sources by Dunlop & Peacock and Jarvis & Rawlings, in line with the expecta tions of unified schemes.