Photometric redshifts for quasars from WISE-PS1-STRM

Three-dimensional wide-field galaxy surveys are fundamental for cosmological studies. For higher redshifts (z greater than or similar to 1.0), where galaxies are too faint, quasars still trace the large-scale structure of the Universe. Since available telescope time limits spectroscopic surveys, photometric methods are efficient for estimating redshifts for many quasars. Recently, machine-learning methods are increasingly successful for quasar photometric redshifts, however, they hinge on the distribution of the training set. Therefore, a rigorous estimation of reliability is critical. We extracted optical and infrared photometric data from the cross-matched catalogue of the WISE All-Sky and PS1 3 pi DR2 sky surveys. We trained an XGBoost regressor and an artificial neural network on the relation between colour indices and spectroscopic redshift. We approximated the effective training set coverage with the K-nearest neighbours algorithm. We estimated reliable photometric redshifts of 2 562 878 quasars which overlap with the training set in feature space. We validated the derived redshifts with an independent, clustering-based redshift estimation technique. The final catalogue is publicly available.