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

TL;DR

This study analyzes Gaia's proper motions of AGNs and quasars, revealing that a significant fraction are double or multiple sources, including gravitational lenses, which impacts the inertial reference frame and our understanding of quasar structure.

Contribution

It identifies and quantifies the prevalence of double and multiple AGNs among Gaia quasars using astrometric data, and provides a list of candidate systems and lenses, advancing knowledge of quasar multiplicity.

Findings

At least 0.17% of AGNs have genuinely perturbed proper motions.

Approximately 29% of high proper motion quasars are doubles or gravitational lenses.

At least 0.33% of optical quasars are genuinely double or multiply imaged.

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 (VIM) effect, and a smaller fraction may be chance alignments with foreground stars causing weak gravitational lensing.