Odd Radio Circles Modeled by Shock-Bubble Interactions

TL;DR

This study tests the hypothesis that Odd Radio Circles (ORCs) arise as synchrotron-emitting vortex rings produced by Richtmyer-Meshkov instabilities when shocks interact with fossil radio lobes in the intergalactic medium. Using 3D magnetohydrodynamic simulations with a scale-free treatment of radiative cooling, the authors explore Mach numbers in the range $1.4\le M \le 16$, initial bubble radii of $R_0\sim140$–$250$ kpc, and ambient conditions typical of the outskirts of galaxy groups. They find that moderate shocks with $M\approx 2$–$4$ reproduce the observed ring morphologies, polarization, and spectral properties, with ring ages of roughly $70$–$200$ Myr and initial energies $E_{\rm tot}\sim10^{57}$–$10^{59}$ erg; the model predicts a redshift-agnostic formation scenario since the ring can be observed without the host galaxy being centered on the ring. The results imply ORCs reside in low-density environments and likely trace re-energized fossil radio lobes, offering testable predictions through aspect-ratio–polarization correlations and motivating higher-resolution polarization observations and simulations to further constrain the physics of these enigmatic objects.

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.

Figures (13)

• Figure 1: Schematic illustration of the shock-bubble interaction setup mapped to a realistic galaxy group environment. (Before) A planar shock propagates through the ambient medium and hits a fossil radio bubble of diameter $D_{\rm bub}\sim300$ kpc embedded within a group halo of virial radius $R_{\rm vir}\sim1$ Mpc. (After) The interaction generates vorticity, producing a vortex-ring-like structure that can give rise to a ring-like radio morphology in projection. The eye symbol indicates the line of sight. The schematic is not to scale except where indicated and is intended to illustrate geometry rather than detailed dynamics.
• Figure 2: Synthetic low-frequency synchrotron intensity (Stokes I) for the fiducial Mach 4 vortex ring simulation, shown face-on at eight evolutionary stages (a-h). Color scale: Logarithmic, unitless normalized intensity (with each panel scaled $I/I_{\text{max}}$), using the viridis colormap. Both rows share the same normalized intensity range ($10^{-6}$ to $1$). Spatial coordinates: Axes in units of bubble radius ($x/R_{\rm bubble}$, $y/R_{\rm bubble}$), spanning $\pm2R_{\rm bubble}$. The central bubble is positioned at (0,0). Temporal evolution: Times in shock crossing units ($t_{\rm cross} = R_{\rm bubble}/v_{\rm s}$): (a) $0.86 t_{\rm cross}$. (b) $2.15 t_{\rm cross}$, (c) $3.44 t_{\rm cross}$, (d) $6.88 t_{\rm cross}$, (e) $9.45 t_{\rm cross}$, (f) $17.18 t_{\rm cross}$, (g) $48.53 t_{\rm cross}$, (h) $52.82 t_{\rm cross}$.
• Figure 3: B-field maps of the fiducial Mach 4 simulation at eight evolutionary stages (a-h), corresponding to the intensity sequence in Fig. \ref{['fig:Intensity']}. Color scale: Normalized intensity ($I/I_{\text{max}}$) using a 'Blues' colormap (darker shades indicate brighter emission). Vectors: magnetic field orientation (derived by rotating polarization E-vectors by $90^\circ$), with length proportional to polarization fraction $p = \sqrt{Q^2 + U^2}/I$. Vectors are sampled at random pixels where $I > 0.01 I_{\text{max}}$.
• Figure 4: Top to bottom: Five observables as function of time for our fiducial Mach 4 simulation: ring width, ring radius, (the aspect ratio,) from the original simulation images and the convolved images, polarization fraction, B-field angle, flux density (normalized by initial flux density) and magnetic energy (normalized by the initial magnetic energy) from the original simulation images. The dashed curves show the same diagnostics for images smoothed to the same effective image resolution as ORC5.
• Figure 5: Radial intensity profile (normalized by the peak intensity) at time $t=17.18t_{\rm cross}$ (time stamp f in Fig. \ref{['fig:radialplots']}.) Shown are the raw intensity profile (dotted blue curve) and the same radial intensity profile of an image smoothed to the same effective resolution as ORC5 (solid blue curve). Also shown are the radius at the peak intensity (dashed green line) and the locations of the FWHM (dashed purple lines).