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

Abstract

Gaia used a large sample of photometrically selected active galactic nuclei (AGNs) and quasars to remove the residual spin of its global proper motion system in order to achieve a maximally inertial reference frame. A small fraction of these reference objects have statistically significant astrometric proper motions in Gaia EDR3. We compile a source sample of 105,593 high-fidelity AGNs with accurate spectroscopically determined redshifts above 0.5 from the SDSS and normalized proper motions below 4. The rate of genuinely perturbed proper motions is at least 0.17%. A smaller high completeness sample of 152 quasars with excess proper motions at a confidence level of 0.9995 is examined in detail. Pan-STARRS images and Gaia-resolved pairs reveal that 29% of the sample are either double sources or gravitationally lensed quasars. An Anderson–Darling test on parameters of a smaller high-reliability sample and their statistical controls reveals 17 significant factors that favor multiplicity and multi-source structure as the main cause of perturbed astrometry. Using a nearest-neighbor distance statistical analysis and counts of close companions in Gaia on a much larger initial sample of AGNs, an excess of closely separated sources in Gaia is detected. At least 0.33% of all optical quasars are genuinely double or multiply imaged. We provide a list of 44 candidate double or multiple AGNs and four previously known gravitational lenses. Many proper motion quasars may be more closely separated, unresolved doubles exhibiting the variability imposed motion effect, and a smaller fraction may be chance alignments with foreground stars causing weak gravitational lensing.

VaDAR: Varstrometry for Dual AGN Using Radio Interferometry

“…Limiting to sources with AENS >5 ensured a highly statistically significant measurement of excess astrometric noise in the Gaia astrometric solution for each target. As Gaiaʼs astrometry is inherently optimized for compact/ unresolved sources (Makarov et al 2012;Makarov & Secrest 2022), quasars with large, extended host galaxies will exhibit spurious astrometric excess noise, as Gaiaʼs scanning window (∼18 pixels along the scan by ∼12 pixels across the scan for the main astrometric field chips; Gaia Collaboration et al 2016) is filled by the host galaxy Makarov & Secrest (2022), Souchay et al (2022). This can be mitigated by making a cut on redshift, which we do here, of z > 0.5 (where 1″ is ∼6.1 kpc).…”

Radio-optical Reference Catalog, Version 1

“…The wide search radius was meant to help us to identify potentially problematic sources in crowded areas and confusion sources because some of the radio-loud quasars do not have detectable optical counterparts, especially at low Galactic latitudes. Double quasars and close optical pairs with field stars generate significant astrometric perturbations, including bogus proper motions or gross errors that are not captured by the formal uncertainties (Makarov et al 2017a;Makarov & Secrest 2022) in the Gaia data. The impact of chance and genuine neighbors depends on the relative brightnesses and the angular separations of the sources.…”

“…In nearby galaxies, the offsets are larger and easier to detect, showing a range of possible causes, including the presence of dust lanes and asymmetries affecting the optical centroiding (Skipper & Browne 2018). However, caution is needed when interpreting Gaia astrometry in extended objects (e.g., Souchay et al 2022;Makarov & Secrest 2022). Nonetheless, a significant fraction of AGNs listed in the ICRF3 S/X catalog (including defining radio sources) reside in nearby galaxies (Makarov et al 2012).…”

“…A number of factors that can perturb the Gaia astrometric measurements of quasars and AGNs have been identified in the literature (e.g., Makarov et al 2012). Dual quasars and optical pairs with field stars are associated with detectably elevated measured proper motions (Souchay et al 2022;Makarov & Secrest 2022). However, the main perturbation of Gaia astrometric and photometric measurements for the S/X sample comes from the extended character of the host galaxies at low redshift.…”

“…The Gaia DR3 data processing pipeline is not adjusted to treat extended images, which in the case of nearby AGNharboring galaxies are often asymmetric or misaligned due to the obscuring dust structures and merger morphology. The astrometric degradation caused by the extended structures at z < 0.5 is strongly correlated with the photometric excess parameter phot_bp_rp_excess_factor (Makarov & Secrest 2022). Spectroscopic redshifts are only known for a fraction of the S/X sample (1014/4536 = 22%, Sexton et al 2022), so we rely in this study on the phot_bp_rp_ex-cess_factor parameter to clean our working sample of the perturbed nearby sources.…”