Paper detail

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

Compact symmetric objects (CSOs) trace the earliest phases of radio-galaxy growth; however, robust classification is difficult when radio cores are weak or invisible. 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 VLBI to reliably locate the central engine and classify CSOs. We analysed 40 literature CSO candidates by overlaying Gaia 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. Our method yields 20 confirmed CSOs. The confirmed CSOs show low integrated variability, slow hotspot advance speeds, and kinematic ages of 20-2000 yr. High-power CSOs tend to be larger and host faster hotspots, while many low-power systems remain sub-kiloparsec and environmentally confined. 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.