A Population Synthesis Model for Odd Radio Circles Produced by Shock-Bubble Interactions

Abstract

Odd Radio Circles (ORCs) are edge-brightened rings of diffuse radio emission of roughly arcminute size whose physical origin remains debated. One proposed mechanism is vortex formation through Shock-Bubble interaction (SBI), in which a merger or accretion shock re-energizes a fossil radio lobe into an edge-brightened ring. We test whether this mechanism can reproduce the observed ORC abundance using a Monte Carlo population-synthesis model that combines the galaxy stellar mass function, the radio-AGN incidence relation, dark-matter halo merger histories, and the inflation and buoyant survival of AGN-inflated bubbles, with synthetic radio fluxes and ring geometries drawn from a grid of magnetohydrodynamic SBI simulations. Applying survey-specific selection to the ASKAP-EMU Pilot Survey (944 MHz) and the LOFAR Two-metre Sky Survey (LoTSS; 144 MHz), we predict $2.7^{+3.9}_{-1.6}$ detectable ORCs in the EMU Pilot, matching the three found there, and $268^{+331}_{-148}$ in LoTSS, of which $7.5^{+9.5}_{-4.2}$ have a counterpart at 1.4 GHz. The predicted hosts are mostly central galaxies in group-scale halos. The predicted rings are physically smaller and lie at lower redshifts than the galaxies identified as hosts of the known ORCs, suggesting that SBI produces mainly the nearby part of the ORC population, or that some ORCs are associated with the wrong host. The population synthesis model also predicts a large population of low surface brightness steep-spectrum ORCs in the LoTSS band, implying a large hidden population and predicting that deeper low-frequency surveys should uncover many more ORCs than are currently known. These results indicate that shock-bubble interactions can account for at least part of the observed incidence of ORCs.