Spectroscopy of QUBRICS quasar candidates: 1672 new redshifts and a golden sample for the Sandage test of the redshift drift

The QUBRICS (QUasars as BRIght beacons for Cosmology in the Southern hemisphere) survey aims at constructing a sample of the brightest quasars with z ? 2.5, observable with facilities in the Southern Hemisphere. QUBRICS makes use of the available optical and IR wide-field surveys in the South and of Machine Learning techniques to produce thousands of bright quasar candidates of which only a few hundred have been confirmed with follow-up spectroscopy. Taking advantage of the recent Gaia Data Release 3, which contains 220 million low-resolution spectra, and of a newly developed spectral energy distribution fitting technique, designed to combine the photometric information with the Gaia spectroscopy, it has been possible to measure 1672 new secure redshifts of QUBRICS candidates, with a typical uncertainty of s(z) = 0.02. This significant progress of QUBRICS brings it closer to (one of) its primary goals: providing a sample of bright quasars at redshift 2.5 < z < 5 to perform the Sandage test of the cosmological redshift drift. A Golden Sample of seven quasars is presented that makes it possible to carry out this experiment in about 1500 h of observation in 25 yr, using the ANDES spectrograph at the 39m ELT, a significant improvement with respect to previous estimates.

Automated summary

The QUBRICS survey aims to construct a sample of the brightest quasars with z > 2.5 observable in the Southern Hemisphere. Utilizing optical and IR wide-field surveys and Machine Learning techniques, QUBRICS generates thousands of bright quasar candidates, with only a few hundred confirmed through follow-up spectroscopy. By utilizing the Gaia Data Release 3 and a newly developed spectral energy distribution fitting technique, QUBRICS has measured 1672 new secure redshifts of candidates, with a typical uncertainty of s(z) = 0.02. This progress brings QUBRICS closer to its goal of providing a sample of bright quasars for cosmological redshift drift experiments.