The Parkes quarter-Jansky flat-spectrum sample 3. Space density and evolution of QSOs

TL;DR

This paper investigates the space density and evolution of QSOs in a radio-selected sample, revealing a significant decline in their numbers beyond redshift 3 and discussing implications for cosmic reionization and dust obscuration.

Contribution

It introduces a model-independent V_max method to detect QSO density decline at high redshift and clarifies previous selection biases affecting luminosity function interpretations.

Findings

Significant decline in QSO space density beyond z=3 (>4 sigma)

Evolution in luminosity density from z=0 to z~1

Minimal influence of QSOs on reionization epoch

Abstract

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 ~ 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.