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

J. V. Wall, Carole Jackson+3 more|ArXiv.org|Aug 6, 2004

Astronomy and Astrophysical ResearchPhysics and Astronomy76 citations

TL;DR

This paper analyzes the Parkes quarter-Jansky flat-spectrum QSO sample to study space density and evolution, using a model-independent $V_{\rm max}$ test to confirm a significant decline in QSO space density beyond $z>3$ at $>4\sigma$ significance. The results show that the high-redshift cutoff is real and not due to dust obscuration, with evolution patterns in radio-loud QSOs closely matching those in X-ray and optically selected QSOs.

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.

Motivation & Objective

• To determine the space density and redshift evolution of flat-spectrum QSOs in the Parkes quarter-Jansky sample.
• To test whether the observed high-redshift cutoff in QSO space density is real or an artifact of dust obscuration.
• To develop a model-independent method to assess evolution beyond $z=3$ using the $V_{\rm max}$ test.
• To compare the evolution of radio-loud QSOs with that of X-ray and optically selected QSOs and star formation rates.
• To evaluate the role of radio-loud QSOs in the reionization epoch ($6<z<17$).

Proposed method

• Applied the $V_{\rm max}$ test to each QSO in the sample to predict the expected number of sources at higher redshifts under uniform space density.
• Used a model-free, robust statistical approach that avoids assumptions about luminosity evolution or density functions.
• Incorporated all QSOs in the sample, not just the most luminous, to improve statistical power and reduce bias.
• Conducted a bootstrap analysis (1000 resamples) to estimate uncertainties in the fitted evolution curve.
• Compared the derived evolution with results from X-ray (Chandra, XMM-Newton, ROSAT) and optical (SDSS) surveys.
• Assessed the impact of non-contemporaneous flux-density measurements, identifying them as a major source of bias.

Experimental results

Research questions

• RQ1 Is the observed decline in QSO space density at $z>3$ statistically significant and real, or an artifact of selection effects?
• RQ2 Does the high-redshift cutoff in the radio-loud QSO population reflect intrinsic evolution or dust obscuration?
• RQ3 How does the evolution of radio-loud QSOs compare with that of X-ray and optically selected QSOs?
• RQ4 To what extent does the space density evolution of QSOs align with the evolution of the star formation rate?
• RQ5 What is the role of radio-loud QSOs in the reionization epoch, given their high luminosity and potential impact on the intergalactic medium?

Key findings

• A significant decline in QSO space density at $z>3$ is confirmed with a significance level of $>4\sigma$ using the $V_{\rm max}$ test.
• The form of the high-redshift evolution in radio-loud QSOs closely matches that observed in X-ray and optically selected QSOs.
• The decline is not due to dust obscuration, as radio-selected samples are unaffected by dust absorption.
• The space density of radio-loud QSOs shows a rapid increase from $z=0$ to $z=1$, flattens between $z=1$ and $z=2.5$, and declines beyond $z=3$.
• The evolution pattern diverges from the star formation rate beyond $z \sim 1.5$, with AGN space density dropping sharply while SFR remains relatively flat to $z>6$.
• Non-contemporaneous flux-density measurements introduce a severe bias that can suppress the apparent redshift cutoff, highlighting the need for timely data in such analyses.