Abstract
Context. Thanks to the cross-identification between the Gaia EDR3 (E-Data Release 3) catalogue and the quasars’ compiled catalogue LQAC-5 (Large Quasar Astrometric Catalogue-5), accurate astrometric parameters as well as photometric measurements were available for a set of 416 113 quasars. Aims. After analysing the astrometric and photometric properties of these quasars,we concentrate our study on objects characterized by significant proper motion, which contradicts the postulate that they are fixed in the celestial sphere. Methods. We derived statistical properties of the positions, parallaxes, and proper motions of the quasars cross-matched with Gaia EDR3. We carried out a specific investigation of quasars with a proper motion larger than 10 mas yr−1, and used colour–colour diagrams in mid-infrared to discriminate these quasars from stellar contamination. We discuss, in detail, a few cases where the abnormal proper motion could be caused by misidentifications. Results. We note that 43% of LQAC-5 quasars were not detected by Gaia EDR3, mainly because of the cut-off magnitude at G = 21. The comparison between r(G) (Gaia) and r (SDSS) shows a normal distribution. The normalized distributions of parallaxes and proper motions are identical to that of the Gaia CRF2, as well as the distribution of the GBP − GRP colour index. We have identified 937 LQAC-5 objects (0.27% of the sample) with Gaia EDR3 proper motion which is significant at the 5σ level. We have clearly shown that below z = 0.35, the integrated magnitude G(r) obtained from SDSS spectra is smaller than G (Gaia), leading to the statement that the quasars are generally resolved under this threshold, which should lead to a bad determination of proper motion. Moreover, to discriminate quasars from stellar contamination, we have plotted them in a mid-infrared colour–colour diagram. Finally, we have made a final selection of 41 objects with μ > 10 mas yr−1, satisfying drastic constraints, which makes them potential candidates for considering their proper motion as real. Conclusions. In this paper we carried out a complete analysis of the quasars of the LQAC-5 which were cross-identified with Gaia EDR3 ; this serves as an alternative and complementary study with respect to Gaia CRF2, involving a different population of quasars. We selected a set of 41 quasars with a proper motion exceeding 10 mas yr−1, which can be considered as very high for objects which are a priori fixed in the celestial sphere.
Highlights
Astrometry is the astronomical discipline concerned with the accurate measurement of the positions of celestial objects as well as with the study of their changes
In the same way as Mignard et al (2018), we investigate the astrometric quality of the sample of 416 113 LQAC5 quasars cross-identified with Gaia Early Data Release 3 (EDR3)
In this paper we have constructed an improved version of the LQAC5 catalogue (Souchay et al 2019), which we called LQAC5+, by matching its total sample of 592809 quasars with the Gaia EDR3 catalogue (Brown et al 2018) This process, with a search radius of 1", led to 416113 cross-matched objects, which correspond to 70.2 % of the totality
Summary
Astrometry is the astronomical discipline concerned with the accurate measurement of the positions of celestial objects as well as with the study of their changes. With that aim, Mignard et al (2018) constructed the Gaia-CRF2, which can be considered as the first optical realization of the nonrotating CRF at sub-milliarcsecond levels of accuracy For this purpose, they used the complete set of astrometric measurements, that bis to say the positions, parallaxes, and proper motions for a sample of more than 550 000 sources coming from the second release of the Gaia DR2 catalogue (Brown et al 2018). Taking the advantages of this new EDR3 release into account for this study, we carried out a cross-identification between LQAC-5 and EDR3, which we call LQAC-5+ It contains exactly the same objects as LQAC-5 but it includes the new values coming from EDR3 concerning the celestial coordinates, the parallax, the proper motion components, the G,GBP, and GRP photometry, together with their respective uncertainty.
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