Models of radio source evolution - II. The 2700-MHz source count

Our technique for deriving cosmological evolution from source counts and identifications is applied to data at 2700 MHz. The analysis is carried out on the assumption that two populations with different evolutionary behaviours appear in surveys at this frequency: ‘steep-spectrum’ sources with extended radio structures, and ‘non-steep-spectrum’ sources with compact structures, the majority of which are identified with QSOs. The 2700-MHz data add constraints to the evolution deduced for the ‘steep-spectrum’ sources from low-frequency data; in particular, of the two types of model obtained in our analysis of the 408-MHz data, only one now appears tenable. The present results for ‘non-steep-spectrum’ sources agree with the results from luminosity – volume tests on samples of ‘flat-spectrum’ QSOs – the change in space density with epoch appears less dramatic than for the powerful radio sources with steep spectra and extended radio structures