![]() ![]() The astrometric properties of the EDR3 quasars show that no significant residuals are found globally ( Liao et al., 2021a, b), which provides us with a unique opportunity to select abnormal quasars in EDR3. The systematic errors in EDR3 have been greatly improved compared with DR2. In Gaia Early Data release 3 (EDR3), the AGN catalog, which contains 1,614,173 sources, is obtained by cross-matching with 17 external AGN catalogs. (2018) selected 556,869 quasars from the third International Celestial Sphere Reference Frame (ICRF3) and AllWISE AGN catalog ( Secrest et al., 2015) to establish the Gaia-CRF2 (see also Mignard et al., 2018). With the accurate position data and a large number of identified quasars, Gaia has been committed to establishing its own optical non-rotating celestial reference frame (CRF) ( Mignard et al., 2018). At present, Gaia has provided high-precision astrometric data for more than 1.8 billion sources in the G magnitude range from 3 to 21 mag ( Lindegren et al., 2021). Gaia is an astrometric satellite launched by the European Space Agency (ESA) on 19 December 2013 ( Prusti et al., 2016). At present, most of the known dual AGN are at low redshifts or have large physical separation ( >20kpc), and only several known small-separation dual quasars are at high redshifts ( Chen et al., 2022), while Gaia’s high-precision astrometric data has not been seriously considered. These dual quasars are precursors of the binary supermassive black holes, which play an important role in the study of galaxy evolution and gravitational waves (GWs). (2019) emphasize that quasars with significant astrometric noises may be dual quasars. These abnormal quasars have large proper motions or significant astrometric noises, which means that they are not suitable to be used to establish the celestial reference frame. Among these confirmed quasars, some spectroscopically identified quasars show abnormal astrometric characteristics in the Gaia high-precision astrometric observation ( Wu et al., 2021). The total number of identified quasars has exceeded one million, and these quasars have been collected and compiled into various catalogs (see, e.g., Véron-Cetty and Véron 2010 Souchay et al., 2019 Liao et al., 2019 Flesch 2021). A large number of quasars have also been identified through astrometry and mid-infrared methods (see, e.g., Secrest et al., 2015 Guo et al., 2018). There are many surveys concerning the identification of quasars such as the large Bright Quasar Survey ( Hewett et al., 1995), the 2DF Quasar Redshift Survey (2QZ, Croom et al., 2004), the quasars from Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, Luo et al., 2012) and Solan Digital Sky Survey (SDSS, Pâris et al., 2018 Lyke et al., 2020). On the other hand, quasars are also a critical pathway to explore the evolution and mergers of galaxies in astrophysics ( Begelman et al., 1980 Shen et al., 2021). In astrometry, a large number of evenly distributed quasars can be used to establish a celestial reference frame ( Ma, 1997 Ma et al., 2009 Mignard et al., 2018 Charlot et al., 2020) because they have almost zero proper motions and point-like shapes. Since the discovery of the first quasar in 1963 ( Schmidt, 1963), this type of extremely distant active galactic nuclei (AGN) has gradually become the focus of astronomical research. ![]() They are potential candidates of quasar pairs. We obtain two catalogs containing 155 sources and 44 sources, respectively. SDSS ASTROMETRY ONLINE SERIESWe propose a series of criteria for selecting abnormal quasars based on Gaia astrometric data. In this work, we aim to find these quasar candidates from Gaia EDR3 astrometric data combining with Sloan Digital Sky Survey (SDSS) spectroscopic data to provide a candidate catalog to the science community. These quasars may come with astrometric jitter detectable with Gaia data, or significant changes in the position at different wavelengths. However, some special quasars may have abnormal astrometric characteristics, such as quasar pairs, lensed quasars, AGNs with bright parsec-scale optical jets, which are scientifically interesting objects, such as binary black holes. Given their extremely large distances and small apparent sizes, quasars are generally considered as objects with near-zero parallax and proper motion. 2School of Astronomy and Space Sciences, University of Chinese Academy of Sciences, Beijing, China.1Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai, China.Qi-Qi Wu 1,2, Shi-Long Liao 1,2*, Xiang Ji 1,2, Zhao-Xiang Qi 1,2, Zhen-Ya Zheng 1,2, Ru-Qiu Lin 1,2, Ying-Kang Zhang 1,2 and Tao An 1,2 ![]()
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