Odd Radio Circles Modeled by Shock-Bubble Interactions

TL;DR

This study models Odd Radio Circles as shock-induced vortex rings formed by the Richtmyer-Meshkov instability, using 3D simulations to match observed properties and explore their origins in low-density environments.

Contribution

We introduce a novel RMI-based model for ORCs, incorporating new cooling methods and demonstrating consistency with observed sizes, energies, and polarization features.

Findings

Shock strengths of Mach 2-4 fit observational data.

Initial bubble sizes range from 140 to 250 kpc.

ORCs are likely in low-density environments like galaxy group outskirts.

Abstract

The physical nature and origins of the newly discovered class of Odd Radio Circles (ORCs) remain unclear. We investigate a model whereby ORCs are synchrotron-emitting vortex rings formed by the Richtmyer-Meshkov instability (RMI) when a shock interacts with a low-density fossil radio lobe in the intergalactic medium using 3D magnetohydrodynamic simulations. These rings initially exhibit oscillatory behavior that damps over time. We implement a new method to model Inverse-Compton cooling and synchrotron cooling at high frequencies in a scale-free manner, enabling us to test a wide range of model parameters against the observational constraints. We find that shock strengths of Mach 2-4 are consistent with the data, as expected in accretion, merger-driven, or active galactic nuclei-driven shocks. We find that the initial size of the bubbles required to explain the rings ranges from 140 to 250 kpc, with initial energy in the bubble of order $10^{57}-10^{59}$ erg, consistent with fossil lobes inflated by moderately powerful radio galaxies. Derived ambient pressures and densities place ORCs in low density environments, such as the outskirts of galaxy groups with ages of order 70-200 Myr. Our synthetic radio maps match the polarization properties of ORC1 and predict a dependency of the tangential magnetic field angle on the aspect ratio of ORCs. A key distinguishing trait of the RMI-driven vortex ring model is that it does not require the ORC to be centered on its host galaxy and is therefore redshift agnostic.