Estimating the completeness of the QUBRICS survey with 3501 quasi-stellar object redshifts from Gaia DR3 spectra

¹ INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo, 11, 34143 Trieste, Italy
² IFPU – Institute for Fundamental Physics of the Universe, via Beirut 2, 34151 Trieste, Italy
³ INFN – National Institute for Nuclear Physics, via Valerio 2, 34127 Trieste, Italy
⁴ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁵ Cerro Tololo Inter-American Observatory/NSF NOIRLab, Casilla 603, La Serena, Chile
⁶ Dipartimento di Fisica dell’Università di Trieste, Sezione di Astronomia, Via G.B. Tiepolo, 11, 34143 Trieste, Italy
⁷ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁸ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
⁹ Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4150-007 Porto, Portugal
¹⁰ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹² Scuola Normale Superiore Piazza dei Cavalieri, 7, 56126 Pisa, Italy
¹³ SISSA – International School for Advanced Studies, Via Bonomea 265, 34136 Trieste, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 17 November 2025
Accepted: 1 March 2026

Abstract

Context. Quasi-stellar objects (QSOs) are essential for investigating the structure and evolution of the Universe. 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.

Aims. This study aims to quantify, using an independent QSO sample, the completeness and recall of the QUBRICS QSO selection methods, based on extreme gradient boosting (XGB) and probabilistic random forest (PRF) techniques, since completeness is a fundamental metric for ensuring the statistical robustness of QSO-based cosmological investigations.

Methods. We analyzed a subset of Gaia DR3 sources (G < 18.25, |b| > 25 deg, negligible parallax and proper motion) with low-resolution spectra, from which we obtained 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. Similarly, 675 QSOs with z > 2.5 were within the PRF dataset footprint, including 69 unclassified objects.

Results. The XGB correctly identified as candidates 136 (89%) of the 152 QSOs with z > 2.5 listed in the XGB dataset as unclassified objects. The PRF correctly identified as candidates 46 (66%) of the 69 QSOs with z > 2.5 listed in the PRF dataset as unclassified objects.

Conclusions. These findings confirm the high efficiency of the QUBRICS selection methods (recall = 89%) and provide the completeness estimate for spectroscopically confirmed QSOs (82%), which is necessary for cosmological studies that use QUBRICS data. This work also provides reliable redshifts for 1223 new QSOs (median redshift z = 2.1 and magnitude G = 17.8), which will help improve the performance of future selections.