Quasars with large proper motions: A selection from the LQAC-5 catalogue combined with Gaia EDR3

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 416113 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/y, 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 G BP − G RP 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/y, 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/y, which can be considered as very high for objects which are a priori fixed in the celestial sphere.

Optical–Radio Position Offsets Are Inversely Correlated with AGN Photometric Variability

“…The results of this process are given in Table 1. The offsets and parallax uncertainty scaling factor are generally consistent with previous literature estimates using large quasar samples (Souchay et al 2022;Makarov & Secrest 2022), but the proper motion scaling factors, about a factor of 1.14, are significantly larger than the literature estimates of about 1.06 (see also Gaia Collaboration et al 2021). The value of 1.06 from the literature was obtained using much larger and heterogeneous samples of AGNs and quasars, namely Gaia-CRF3, LQAC-5 (Souchay et al 2019), and mid-IR AGNs (Secrest et al 2015;Gaia Collaboration et al 2021;Makarov & Secrest 2022;Souchay et al 2022, respectively), suggesting that 1.06 is a more accurate estimate of the degree to which Gaia EDR3/DR3 astrometric uncertainties are underestimated due to data-processing issues.…”

“…To avoid confounding factors that may induce optical-radio offsets, such as spurious astrometry due to source extent or source multiplicity (e.g., dual AGNs), I applied several quality cuts. First, I required that the BP/RP excess factor (phot_bp_rp_excess_factor) be less than 2, which helps eliminate extended sources, such as those at lower redshifts (Souchay et al 2022;Makarov & Secrest 2022). Second, I required that the significance of any astrometric excess noise (astrometric_excess_noise_sig) be less than 2.…”

“…Further details of these parameters are given in the Gaia DR3 data model documentation. 3 Finally, apparently significant parallaxes and proper motions in bona fide quasars are an indication of multiplicity, such as dual AGNs or gravitational lenses (Souchay et al 2022;Makarov & Secrest 2022), so they should be removed. Note that calculation of the uncertainty-normalized parallaxes and proper motions first requires correction for the zero-point offsets and uncertainty scaling factors.…”

Quasars with Proper Motions and the Link to Double and Multiple AGNs

“…The significances of these zero-point offsets are 25σ, 4.5σ, and 2.6σ, respectively. Our empirically determined error correction factors are consistent with the standard deviation of normalized proper motions of 1.063 determined by Gaia Collaboration et al (2021b), as well as with the offsets determined in Souchay et al (2022). We show the absolute values of these corrected, error-normalized quantities as a function of redshift in Figure 1, demonstrating that AGNs with redshifts below 0.5 have systematically elevated significances.…”

Quasars with Proper Motions and the Link to Double and Multiple AGNs

“…The significances of these zero-point offsets are 25σ, 4.5σ, and 2.6σ, respectively. Our empirically determined errorcorrection factors are consistent with the standard deviation of normalized proper motions of 1.063 determined by Gaia Collaboration et al (2021b), as well as with the offsets determined in Souchay et al (2022). We show the absolute values of these corrected, error-normalized quantities as a function of redshift in Figure 1, demonstrating that AGNs with redshifts below 0.5 have systematically elevated significances.…”