Finding Quasars behind the Galactic Plane. IV. Candidate Selection from Chandra with Random Forest

The Astrophysical Journal, vol. 1002, pp. 76

Abstract

Abstract Quasar samples remain severely incomplete at low Galactic latitudes because of strong extinction and source confusion. We conduct a systematic search for quasars behind the Galactic plane using X-ray sources from the Chandra Source Catalog (CSC), combined with optical data from Gaia Data Release 3 and mid-infrared data from CatWISE2020. Using spectroscopically confirmed quasars and stellar-type objects from datasets including DESI, the Sloan Digital Sky Survey, and LAMOST, we apply a random forest classifier to identify quasar candidates, with stellar contaminants suppressed using Gaia proper-motion constraints. Photometric redshifts are estimated for the candidates using a random forest regression model. Applying this framework to previously unclassified CSC sources, we identify 7570 quasar candidates, including 1060 Galactic plane quasar (GPQ) candidates at ∣ b ∣ < 20°, of which 551 are high-confidence candidates. Relative to the previously known GPQ sample, our selected GPQs reach lower optical and X-ray fluxes, improving sensitivity to low-flux GPQs. In addition, both the GPQ candidates and known GPQs display harder X-ray spectra than the all-sky quasar sample, consistent with increased absorption through the Galactic plane. Pilot spectroscopy confirms two high-confidence GPQ candidates as quasars at spectroscopic redshifts of z = 1.2582 and z = 1.1313, and further spectroscopic follow-up of the GPQ sample is underway. This work substantially improves the census of GPQs and provides a valuable target sample for future spectroscopic follow-up, enabling the use of GPQs to refine the reference frames for astrometry and probe the Milky Way interstellar and circumgalactic media with the absorption features of GPQs.

Authors 9

1. Yanli Ai

   Shenzhen Technology University

   Affiliation as printed

   College of Engineering Physics, Shenzhen Technology University, Shenzhen 518118, People’s Republic of China; aiyanli@sztu.edu.cn

2. Y H Zhang

   National Astronomical Observatories

   Affiliation as printed

   National Astronomical Observatories, Beijing 100101, People’s Republic of China

3. Yuming Fu Aachen Leiden Observatory

   Leiden University · University of Groningen · Leiden Observatory

   Affiliation as printed

   Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands

   Leiden Observatory, Leiden University, Einsteinweg 55, 2333 CC Leiden, The Netherlands

4. Xue-Bing Wu

   Peking University

   Affiliation as printed

   Department of Astronomy, School of Physics, Peking University, Beijing 100871, People’s Republic of China

   Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, People’s Republic of China

5. Wenfeng Wen

   Shenzhen Technology University

   Affiliation as printed

   College of Engineering Physics, Shenzhen Technology University, Shenzhen 518118, People’s Republic of China; aiyanli@sztu.edu.cn

6. Jiayuan Zhou

   Shenzhen Technology University

   Affiliation as printed

   College of Engineering Physics, Shenzhen Technology University, Shenzhen 518118, People’s Republic of China; aiyanli@sztu.edu.cn

7. Dexuan Kong

   Shenzhen Technology University

   Affiliation as printed

   College of Engineering Physics, Shenzhen Technology University, Shenzhen 518118, People’s Republic of China; aiyanli@sztu.edu.cn

8. Linfeng Zeng

   Shenzhen Technology University

   Affiliation as printed

   College of Engineering Physics, Shenzhen Technology University, Shenzhen 518118, People’s Republic of China; aiyanli@sztu.edu.cn

9. Heng Wang

   Shenzhen Technology University

   Affiliation as printed

   College of Engineering Physics, Shenzhen Technology University, Shenzhen 518118, People’s Republic of China; aiyanli@sztu.edu.cn