Owl-z: Bayesian tool for selecting z ≳ 7 quasars

¹ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
² Aix Marseille Univ, CNRS, I2M, Marseille, France
³ Aix Marseille Univ, CNRS, LIS, Marseille, France
⁴ Canada-France-Hawai’i Telescope, Waimea, Hawai’i, USA
⁵ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
⁶ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands

^★ Corresponding author: meriam.ezziati@lam.fr

Received: 1 January 2025
Accepted: 2 July 2025

Abstract

This paper presents Owl-z, a Bayesian code aimed at identifying z ≥ 7 quasars in wide-field optical and near-infrared surveys. By construction, the code can also be used to select objects that contaminate the high-z quasar population, such as brown dwarfs and early-type galaxies at intermediate redshifts. The code can be adapted for the selection of high-z galaxies and although it has been tuned to the Euclid Wide Survey, it can be easily adapted to other photometric surveys. The code input data comprise the object’s photometric data and its galactic longitude and latitude, while the code output data are the probabilities of the modelled populations of high-z quasars, brown dwarfs, and early-type galaxies at intermediate redshift. As part of the validation, Owl-z was able to re-identify all spectroscopically confirmed quasars at z ≥ 7, demonstrating the code’s versatility in its application to different photometric catalogues. We analysed the performance of Owl-z, based on a metric combining completeness and purity called F-measure, in the case of Euclid using simulated data in a wide range of redshifts (7 ≤ z ≤ 12) and H-band Euclid magnitudes (18 ≤ H_E ≤ 24.5). The results show that Owl-z reaches full performance for bright sources (H_E ⪅ 22), somewhat independently of redshift. We show that the probability threshold used to select promising quasar candidates can be adjusted after processing to fine-tune the F-measure values for candidates, depending on their magnitude and redshift estimates. We show that for objects brighter than about two magnitudes above the survey detection limit, Owl-z provides a good classification that will facilitate the optimisation of photometric and spectroscopic confirmation campaigns. In conclusion, Owl-z offers a powerful public tool to help select high-z quasars, brown dwarfs, or early-type galaxies at intermediate redshifts in Euclid or other wide-field surveys.