The completeness of the QUBRICS Survey

Historically, their identification has been concentrated in the northern hemisphere, primarily due to the sky coverage of major astronomical surveys. The QUBRICS (QUasars as BRIght beacons for Cosmology in the Southern hemisphere) survey, started in 2019 to address this asymmetry, has identified more than 1300 new bright (i<19.5) high-redshift (2.5<z<6) QSOs in the southern sky.

This study aims to quantify, using an independent QSO sample, the completeness and recall of the QUBRICS QSO selection methods, based on XGB (eXtreme Gradient Boosting) and PRF (Probabilistic Random Forest), since completeness is a fundamental metric for ensuring the statistical robustness of QSO-based cosmological investigations. A subset (G<18.25, |b|>25deg, negligible parallax and proper motion) of Gaia DR3 sources with low-resolution spectra was analyzed, obtaining a sample of 3501 QSOs.To determine how many QSOs were correctly identified as candidates, we crossmatched this independent sample with the datasets used for selection: 894 QSOs with z>2.5 fell within the XGB dataset footprint, of which 152 were unclassified and thus eligible for completeness testing.