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Characterizing the astrometric quality of AGNs in Gaia-CRF3

Shilong Liao, Qiqi Wu, Ye Ding, Qi Xu, Zhaoxiang Qi

TL;DR

The paper addresses establishing a robust, non-rotating optical celestial reference frame (Gaia-CRF3) by characterizing astrometric systematics in Gaia DR3 AGNs. It analyzes parallaxes and proper motions of Gaia EDR3 AGNs, cross-matches with external quasar catalogs, and applies multi-criteria quality assessments to identify reliable reference sources. A key contribution is the introduction of an astrometric quality index based on a TOPSIS multi-parameter evaluation, enabling the separation of high-fidelity AGN anchors from problematic sources, including dual or lensed systems. The results show a global parallax bias of roughly $-20$ to $-30\,\mu\mathrm{as}$ with spatial structure and small color/magnitude trends, and demonstrate that most AGNs provide stable astrometry while a minority require exclusion; the index thus supports robust selection of AGN anchors for Gaia-CRF3.

Abstract

Active Galactic Nuclei (AGNs), owing to their great distances and compact sizes, serve as fundamental anchors for defining the celestial reference frame. With about 1.9 million AGNs observed in Gaia DR3 at optical precision comparable to radio wavelengths, Gaia provides a solid foundation for constructing the next-generation, kinematically non-rotating optical reference frame. Accurate assessment of systematic residuals in AGN astrometry is therefore crucial. In this talk, we analysed the parallaxes and proper motions of Gaia DR3 AGNs to characterize systematic errors and their correlations with various physical and observational properties. A subset of Gaia-CRF3 AGNs exhibits significant astrometric offsets, mainly arising from dual or lensed quasars whose structural variations induce photocenter jitter, mimicking parallax and proper motion. Such sources must be carefully excluded from reference frame construction. To this end, we introduce an astrometric quality index for each source to quantify its astrometric reliability. The results reveal a strong correlation between lower quality index values and increasing errors in position, proper motion, and parallax, demonstrating that the proposed index provides an effective metric for selecting high-fidelity AGNs as primary reference sources.

Characterizing the astrometric quality of AGNs in Gaia-CRF3

TL;DR

The paper addresses establishing a robust, non-rotating optical celestial reference frame (Gaia-CRF3) by characterizing astrometric systematics in Gaia DR3 AGNs. It analyzes parallaxes and proper motions of Gaia EDR3 AGNs, cross-matches with external quasar catalogs, and applies multi-criteria quality assessments to identify reliable reference sources. A key contribution is the introduction of an astrometric quality index based on a TOPSIS multi-parameter evaluation, enabling the separation of high-fidelity AGN anchors from problematic sources, including dual or lensed systems. The results show a global parallax bias of roughly to with spatial structure and small color/magnitude trends, and demonstrate that most AGNs provide stable astrometry while a minority require exclusion; the index thus supports robust selection of AGN anchors for Gaia-CRF3.

Abstract

Active Galactic Nuclei (AGNs), owing to their great distances and compact sizes, serve as fundamental anchors for defining the celestial reference frame. With about 1.9 million AGNs observed in Gaia DR3 at optical precision comparable to radio wavelengths, Gaia provides a solid foundation for constructing the next-generation, kinematically non-rotating optical reference frame. Accurate assessment of systematic residuals in AGN astrometry is therefore crucial. In this talk, we analysed the parallaxes and proper motions of Gaia DR3 AGNs to characterize systematic errors and their correlations with various physical and observational properties. A subset of Gaia-CRF3 AGNs exhibits significant astrometric offsets, mainly arising from dual or lensed quasars whose structural variations induce photocenter jitter, mimicking parallax and proper motion. Such sources must be carefully excluded from reference frame construction. To this end, we introduce an astrometric quality index for each source to quantify its astrometric reliability. The results reveal a strong correlation between lower quality index values and increasing errors in position, proper motion, and parallax, demonstrating that the proposed index provides an effective metric for selecting high-fidelity AGNs as primary reference sources.

Paper Structure

This paper contains 7 sections, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. The Astrometric Properties of Gaia EDR3 revelled by AGNs
  3. AGN-like Objects Used
  4. The Astrometric Properties of Gaia EDR3
  5. AGN Candidates With Abnormal Astrometric Characteristics
  6. The astrometric quality index of AGNs in Gaia
  7. Summary and Conclusions

Figures (3)

  • Figure 1: Maps of the generalized moving mean parallaxes (in mas) for the Gaia EDR3 quasar sample. The maps are shown in a Hammer–Aitoff projection in ecliptic coordinates. Left: Gaia EDR3 FPQ subset. Right: Gaia EDR3 SPQ subset.
  • Figure 2: Four SDSS DR16 quasars images with abnormal astrometric behavior (Left panel).Cumulative distribution histograms of positional errors in right ascension (RA) and declination (Dec) for different quasar subsets. EB1–EB3 represent the subsets of abnormal (or “strange”) AGN candidates identified in this work (middle and right panels).
  • Figure 3: Proper motion (left) and parallax (right) as a function of magnitude for Gaia-CRF3 sources of different quality classes.