Searching for dual/lensed quasar candidates with spectroscopic surveys

TL;DR

This paper presents a systematic spectroscopic survey method to identify dual and lensed quasar candidates, enhancing the discovery of these astrophysically significant objects and providing insights into black hole growth and cosmology.

Contribution

The authors develop a new diagnostic approach using broad emission line profiles to efficiently find dual and lensed quasars in large spectroscopic datasets.

Findings

Identified 30 lensed quasar candidates with similar emission line profiles.

Found 36 dual quasar candidates with different emission line profiles.

Confirmed four candidates through archival HST data, including three known lensed quasars and one new dual quasar candidate.

Abstract

Dual and lensed quasars are valuable astrophysical targets in many aspects. Dual quasars, considered as the precursors of supermassive black hole binaries, can provide crucial insights into how black hole mergers drive the growth of supermassive black holes and influence the evolution of galaxies. Lensed quasars, formed by the gravitational deflection of a background quasar's light by a massive foreground object, can address key cosmological questions, particularly in refining measurements of the Hubble constant. Despite their significance, the number of confirmed dual and lensed quasars remains limited. Here in this work, we propose a systematic search for dual/lensed quasars using broad emission line profile diagnostics. Our parent sample consists of spectroscopic quasars from the Dark Energy Spectroscopic Instrument Early Data Release (DESI EDR) and the SDSS DR17 catalog. We identify 30 lensed quasar candidates with similar broad emission line profiles, as well as 36 dual quasar candidates with different profiles. Cross-matching these 66 targets with the HST archival database, we find four overlapping targets, including three previously reported lensed quasars and one newly identified dual quasar candidate. We estimate the black hole masses for the two cores in the same system. The mass ratios are similar in the lensed quasar scenario but vary widely for dual quasars, consistent with the physical nature of these two types. In particular, we identified a dual quasar candidate with the mass ratio exceeding 100 times. We aim to discover more dual/lensed quasar candidates using our method with the upcoming future spectroscopic surveys.