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

Abstract

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.

Figures (2)

• Figure 1: Flowchart of our sample selection procedure. P&T2000: 2000ApJ...534...90P; A&B2012: 2012ApJ...760...77A; T2016: 2016MNRAS.459..820T. The Radio Fundamental catalogue (RFC): http://astrogeo.org/rfc/2024arXiv241011794P. Gaia DR3: https://www.cosmos.esa.int/web/gaia/data-release-3.
• Figure 2: Parameter distributions of sample of 32 CSO sources combined with model predictions. Black symbols denote low-power CSOs ($P_{\rm rad}<10^{26.5}$ W Hz$^{-1}$) and red symbols represent high-power CSOs. The filled symbols are from the current study and the open symbols are from 2012ApJ...760...77A. Dashed red and black arrows relate to evolutionary trend for high- and low-power source distributions. The dashed blue lines in panels a, c, and f show the general power trend of the groups as a whole. The dashed blue line in panel b shows the expected survival threshold for low power sources. Panel $\bf a$: Dashed red and black arrows denote the predicted $P_{\rm rad} \propto D^{2/3}$ adiabatic-loss-dominated evolution of a source with constant power, and the dashed blue line indicates the predicted power-size distribution: $P_{\rm rad} \propto D^4$. Panel $\bf b$: Dashed red and black arrows show the predicted inverse relationship between radio power and hotspot velocity: $P_{\rm rad} \propto V_{\rm HS}^{-1}$. Panel $\bf c$: Hotspot velocity varies with the projected source size as $V_{\rm HS} \propto D^{-2/3}$. Panel $\bf d$: Dashed arrows show the predicted evolutionary relation of $P_{\rm rad} \propto T^{2/5}$. Panels $\bf e$ and $\bf f$: Dashed arrows indicate the predicted evolution following $V_{\rm HS} \propto T^{-2/5}$ and $D \propto T^{3/5}$. Panels g and h: Relatively flat distribution of spectral index shows a slight steepening toward $\alpha = 1$ with increasing kinematic age, $T,$ and with projected source size, $D$.