Analysis of possible anomalies in the QSO distribution of the Flesch & Hardcastle catalogue

TL;DR

This study critically examines the Flesch & Hardcastle QSO catalogue, revealing overestimated probabilities leading to apparent anomalies in QSO distribution, and clarifies the true nature of these candidates through spectral analysis.

Contribution

It provides an extensive validation of QSO candidate probabilities, correcting previous overestimations and explaining observed distribution anomalies.

Findings

Only 12 of 41 candidates are confirmed QSOs.

Probabilities are overestimated near bright galaxies and for bright objects.

Anomalies are due to probability overestimations, not actual distribution.

Abstract

AIMS. A recent catalogue by Flesch & Hardcastle presents two major anomalies in the spatial distribution of QSO candidates: i/ an apparent excess of such objects near bright galaxies, and ii/ an excess of very bright QSO candidates compared to random background expectations in several regions of the sky. Because anyone of these anomalies would be relevant in a cosmological context, we carried out an extensive analysis of the probabilities quoted in that catalogue. METHODS. We determine the nature and redshift of a subsample of 30 sources in that catalogue by analysing their optical spectra (another 11 candidates were identified from existing public databases). These have allowed us to statistically check the reliability of the probabilities QSO status quoted by Flesch & Hardcastle for their candidates. RESULTS. Only 12 of the 41 candidates turned out QSOs (7 of which have been identified here for the first time). CONCLUSIONS. The probabilities of the QSOs' being the candidates given by Flesch & Hardcastle are overestimated for m_B<17 and for objects projected near (<1 arcmin) bright galaxies. This is the cause of the anomalies mentioned above.