Search for quasar pairs with Gaia astrometric data. II. Photometric redshift prediction with machine learning for the MGQPC catalogue

TL;DR

This paper presents a machine learning approach to estimate photometric redshifts for quasars, enabling efficient identification of quasar pairs for studies of galaxy evolution and black hole growth.

Contribution

The authors develop a novel machine learning framework using CatBoost and FlexZBoost to accurately predict quasar redshifts from multi-wavelength photometry, aiding in quasar pair detection.

Findings

Achieved a normalized median absolute deviation of 0.036 in redshift estimates.

Identified 185 high-probability quasar pair candidates in the MGQPC catalogue.

Confirmed 20 quasar pairs through spectroscopic follow-up.

Abstract

The identification of physically associated kiloparsec-scale quasar pairs is important for understanding galaxy evolution, the growth of supermassive black holes, and their co-evolution with host galaxies. However, their rarity and the high contamination from stellar superpositions and projected alignments require efficient pre-selection methods. We develop a machine-learning framework to produce photometric-redshift point estimates and redshift probability density functions for quasars, with the main goal of identifying high-probability quasar pair candidates in the MGQPC catalogue. We construct two large spectroscopically confirmed quasar samples with multi-wavelength photometry, based on SDSS and DESI Legacy Imaging Surveys data. CatBoost is used for point-estimate photometric-redshift regression, and FlexZBoost is used for full redshift-PDF estimation. The workflow achieves robust performance, with a normalised median absolute deviation of 0.036 and an outlier fraction of 5.6% on the test sample. Applying the trained model to the MGQPC catalogue, we identify 185 high-probability quasar pair candidates based on photometric-redshift consistency. Among them, 20 systems have been subsequently confirmed as genuine physical pairs by independent spectroscopic observations. The resulting MGQPC photometric-redshift catalogue provides a useful resource for future spectroscopic follow-up of quasar pairs and dual supermassive black holes.