2025A&A...704A..93A

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.