Astrometric properties of reference frame sources as a function of redshift

¹ School of Astronomy and Space Science, Nanjing University, Nanjing 210023, China
² Key Laboratory of Modern Astronomy and Astrophysics (Ministry of Education), Nanjing University, Nanjing 210023, China
³ Research Center for Computing, National Research and Innovation Agency, Bandung 40135, Indonesia
⁴ School of Physical Science and Technology, Xinjiang University, Urumqi 830046, China
⁵ University of Chinese Academy of Sciences, Nanjing 211135, China

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 22 December 2025
Accepted: 18 February 2026

Abstract

Context. Previous studies based on the latest realisation of the International Celestial Reference Frame (ICRF3) have suggested a correlation between astrometric properties (such as the radio-optical offset) and redshift for active galactic nuclei (AGNs).

Aims. We extend these investigations by using a large, all-sky sample of approximately 22 000 compact radio sources from the Radio Fundamental Catalogue (RFC) to examine this relationship in a systematic and statistically robust manner.

Methods. We compiled redshifts for about 10 000 RFC sources over the range 0 < z < 5 by combining data from the Dark Energy Spectroscopic Instrument Data Release 1 and the Sloan Digital Sky Survey Data Release 17/19 with additional datasets from the NASA/IPAC Extragalactic Database. Cross-matching with Gaia Data Release 3 yielded a sample of 4068 RFC objects with reliable spectroscopic redshifts and classifications, including galaxies and quasi-stellar objects (QSOs). We analysed the redshift dependence of their radio astrometric properties from very long baseline interferometry (VLBI) and their optical astrometric properties from Gaia.

Results. We find that the VLBI astrometric properties show no significant dependence on redshift within the achieved level of precision. In contrast, several optical astrometric quantities exhibit clear redshift-dependent behaviour. The median absolute radio–optical offsets decrease markedly over 0< z< 0.5, where galaxies dominate the sample, decline more gradually over 0.5 < z< 1.3, and exhibit a mild increase at z > 1.3, where QSOs dominate. Similar behaviour is observed for several Gaia astrometric quantities, including astrometric uncertainties, proper motions, and G magnitudes. These behaviours can be largely explained by the dependence of Gaia astrometric performance on G magnitude and by the evolution of the G magnitude with redshift.