Search for Quasar Pairs with ${\it Gaia}$ Astrometric Data. III. Discovery of 9 dual and projected quasars

Qihang Chen, Zizhao He, Zhuojun Deng, Liang Jing, Xingyu Zhu, Jianghua Wu

First listed 2026-07-02 | Last updated 2026-06-30

Abstract

We report the low-resolution long-slit spectroscopic observations and confirmations of 11 quasar pair candidates, which are selected from the MGQPC catalog presented in the first paper of our series work (hereafter, Paper-I) and the early version of this catalog. The spectroscopic follow-up was carried out with 5 spectrographs equipped on 3 telescopes, and the major discoveries include 6 dual quasars and 3 projected quasars. One of the dual quasars has a high redshift of $\sim$ 3.1. The LQ hypothesis of 3 dual quasars cannot be completely ruled out. We investigated the reason why previous spectroscopic surveys missed several new quasars. We discussed a projected quasar with a wide-separation lensing configuration, as well as two quasar-star projections that mimic the configuration of lensed quasars. The photometric redshifts for the 11 observed candidates were extracted from the second paper of our series work (hereafter, Paper-II) to illustrate their positive role in mitigating contamination from projected quasars and quasar-star projections. We also reviewed and discussed the confirmation strategies for dual and lensed quasar candidates, and outlined future confirmation strategies for them in the context of the era dominated by large-scale spectroscopic and imaging surveys.

Short digest

This paper spectroscopically follows up 11 Gaia-selected quasar-pair candidates from the MGQPC catalog and confirms 9 real quasar pairs: 6 dual quasars and 3 projected quasars. The long-slit spectra identify one dual system at z ~ 3.1 and separate physical pairs from contaminants, including a wide-separation projected quasar with a lens-like configuration and quasar-star look-alikes. A practical result is that some companions were previously missed because of blending, slit geometry, faintness, or survey limitations, while companion photometric redshifts help suppress projected-quasar contamination before follow-up. The main caveat is that a lensed-quasar interpretation cannot yet be fully excluded for three of the nominal dual quasars, making the paper as much about confirmation strategy as about sample growth.

Key figures to inspect

• Figure 2. This is the paper's core discovery figure. The spectra directly establish the six dual quasars through quasar emission-line identifications in both members, and the public-spectrum overlays also make clear why several companions were previously missed because of blending, faintness, or incomplete earlier spectroscopy.
• Figure 3. Include this to show that the follow-up program does not only add dual quasars but also cleanly identifies projected quasars. It is the direct evidence for the three redshift-mismatched quasar projections, including the lens-like wide-separation case emphasized in the paper's discussion.
• Figure 4. This is the most important caveat and interpretation diagnostic. The B/A spectral flux-ratio curves show emission-line-dependent differences that argue against a simple lensing explanation in several systems, while also illustrating why the lensed-quasar hypothesis cannot be fully excluded for three of the nominal duals.
• Figure 5. This later diagnostic captures one of the paper's main practical lessons for future searches. By placing member B in quasar color space, it shows how color information and the companion photometric-redshift estimates from Paper II can reduce contamination from projected quasars before expensive spectroscopic confirmation.
• Figure 6. This figure is worth including because the abstract explicitly highlights quasar-star projections that mimic lensed quasars. The stellar absorption features, including H and the Ca II triplet in the contaminant objects, demonstrate a concrete failure mode for quasar-pair and lens searches and motivate the paper's confirmation-strategy discussion.