Identifying compact symmetric objects with high-precision VLBI and Gaia astrometry

¹ Shanghai Astronomical Observatory, Key Laboratory of Radio Astronomy, CAS, 80 Nandan Road, Shanghai 200030, China
² State Key Laboratory of Radio Astronomy and Technology, A20 Datun Road, Chaoyang District, Beijing, PR China
³ Konkoly Observatory, HUN-REN Research Center for Astronomy and Earth Sciences, Konkoly Thege Miklós út 15-17, H-1121 Budapest, Hungary
⁴ CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, H-1121 Budapest, Hungary
⁵ Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, H-1117 Budapest, Hungary
⁶ Xinjiang Astronomical Observatory, CAS, 150 Science-1 Street, Ürümqi, Xinjiang 830011, PR China
⁷ Netherlands Institute for Radio Astronomy, ASTRON, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands

^⋆ Corresponding author: antao@shao.ac.cn

Received: 28 February 2025
Accepted: 27 September 2025

Abstract

Context. Compact symmetric objects (CSOs) trace the earliest phases of radio-galaxy growth; however, robust classification is difficult when radio cores are weak or invisible.

Aims. We aim to develop and test a Gaia+VLBI approach that utilizes the high-precision optical astrometry of Gaia together with the high-resolution imaging of very long baseline interferometry (VLBI) to reliably locate the central engine and classify CSOs.

Methods. We analysed 40 literature CSO candidates by overlaying Gaia Data Release 3 (DR3) positions on VLBI maps and by examining spectral index distributions, whole-source variability, and hotspot kinematics over up to 25 years. A source is classified as a CSO when the Gaia centroid lies between two steep-spectrum lobes; alignment with one end of the radio structure indicates a core–jet source.

Results. Our method yields 20 confirmed CSOs, ten core–jet sources, and ten ambiguous cases affected by significant optical–radio positional offsets or limited data. The confirmed CSOs show low integrated variability, hotspot advance speeds typically < 0.5 c, where c denotes the speed of light (with a few mildly relativistic cases), and kinematic ages of ≈20 − 2000 yr. Five nearby CSOs show systematic Gaia–VLBI offsets despite the CSO-like morphology, likely reflecting host-galaxy environments and Gaia astrometric systematics. We find a clear dichotomy with radio power: high-power CSOs (P_1.4 GHz > 10^26.5 W Hz⁻¹) tend to be larger and host faster hotspots, while many low-power systems remain sub-kiloparsec and environmentally confined.

Conclusions.Gaia+VLBI registration is a powerful method for CSO classification, especially where radio cores are faint. The observed power-size-velocity-age relations support distinct multiple evolutionary tracks, with high-power CSOs plausibly growing into large radio galaxies, while low-power CSOs appear confined by their host galaxy environments. Taken together, our results indicate that CSO evolution is shaped not only by intrinsic jet power, but also by host–galaxy environment and the duty cycle of the central engine. High-sensitivity observations of low-power CSOs will be crucial to map the full diversity of formation channels and evolutionary pathways of radio galaxies.