The Parkes quarter-Jansky flat-spectrum sample - III. Space density and evolution of QSOs

We analyze the Parkes quarter- Jansky flat- spectrum sample of QSOs in terms of space density, including the redshift distribution, the radio luminosity function, and the evidence for a redshift cutoff. With regard to the luminosity function, we note the strong evolution in space density from the present day to epochs corresponding to redshifts similar to 1. We draw attention to a selection effect due to spread in spectral shape that may have misled other investigators to consider the apparent similarities in shape of luminosity functions in different redshift shells as evidence for luminosity evolution. To examine the evolution at redshifts beyond 3, we develop a model- independent method based on the V-max test using each object to predict expectation densities beyond z = 3. With this we show that a diminution in space density at z > 3 is present at a significance level > 4 sigma. We identify a severe bias in such determinations from using flux- density measurements at epochs significantly later than that of the finding survey. The form of the diminution is estimated, and is shown to be very similar to that found for QSOs selected in X- ray and optical wavebands. The diminution is also compared with the current estimates of star- formation evolution, with less conclusive results. In summary we suggest that the reionization epoch is little influenced by powerful flat- spectrum QSOs, and that dust obscuration does not play a major role in our view of the QSO population selected at radio, optical or X- ray wavelengths.