https://njuastrometry.github.io/en/author/sufen-guo/Sufen GuoHugo Blox Builder (https://hugoblox.com)en-USWed, 01 Apr 2026 00:00:00 +0000https://njuastrometry.github.io/media/icon_hu1909758375025618910.pnghttps://njuastrometry.github.io/en/author/sufen-guo/https://njuastrometry.github.io/en/publication/2026-aa-708-a83/Wed, 01 Apr 2026 00:00:00 +0000https://njuastrometry.github.io/en/publication/2026-aa-708-a83/<p><strong>Sample and analysis</strong></p> <p>Starting from 21,949 compact radio sources in RFC release <code>rfc_2025c</code>, we compile redshifts for 9907 objects using DESI DR1, SDSS DR17/DR19, and literature data. After spectroscopic quality checks and cross-matching with Gaia DR3, the final sample contains 4068 sources, including 934 ICRF3 sources. Spectral classifications identify 784 galaxies, 3151 quasi-stellar objects (QSOs), and 133 sources with stellar or inconsistent classifications.</p> <p><strong>Main results</strong></p> <ul> <li>VLBI positional uncertainties, radio structure indices, and compactness show no significant redshift dependence within the current precision.</li> <li>Median absolute radio-optical offsets decrease markedly over $0&lt;z&lt;0.5$, decline more gradually over $0.5&lt;z&lt;1.3$, and show a mild increase at $z&gt;1.3$.</li> <li>The optical trends largely reflect Gaia&rsquo;s magnitude-dependent astrometric performance and the changing mixture of galaxies and QSOs. Galaxies have larger optical astrometric uncertainties even at a fixed $G$ magnitude, indicating an additional contribution from extended morphology.</li> <li>The nearly constant median error-normalised proper motion of approximately 1.2 at $z&gt;0.5$ is consistent with the observed proper motions being dominated by random measurement uncertainties.</li> </ul> <p>These results help distinguish observational and source-population effects from an intrinsic dependence on redshift when evaluating sources for the optical–radio reference frame connection.</p>