Cosmological Zoom-In Simulation of Odd Radio Circles as Merger-Driven Shocks in Galaxy Groups

Odd Radio Circles (ORCs) are a new class of distinct radio objects that has recently been discovered. The origin of these features is yet unclear because their peculiar properties are a challenge for our current understanding of astrophysical sources for diffuse radio emission. In this work we test the feasibility of major mergers in galaxy groups as a possible formation channel for ORCs. By modeling the assembly of a massive galaxy group with a final virial mass of $M_{200}\sim 10^{13}, \rm M_\odot$ in a magnetohydrodynamic zoom-in simulation with on-the-fly cosmic ray treatment, we are able to derive the X-ray and radio properties of the system self-consistently and compare them to observations. We show that the X-ray properties for the simulated system are agreeing with characteristics of observed galaxy groups in the regarded mass range, legitimating the comparison between the radio properties of the simulated halo and those of observed ORCs. A major merger between two galaxies in the simulation is triggering a series of strong shocks in the circumgalactic medium, which in unison are forming a ring if the line of sight is perpendicular to the merger axis. The shock is rapidly expanding in radial direction and quickly reaches the virial radius of the halo. This formation channel can hence readily explain the morphology and large extent of ORCs. However, the inferred radio luminosity of these features is lower than for observed counterparts, while the degree of polarization seems to be systematically overpredicted by the simulation. Fossil cosmic ray populations from AGN and stellar feedback might be necessary to explain the full extent of the radio properties of ORCs, since diffusive shock acceleration was the only source term for non-thermal electrons considered in this work.

💡 Research Summary

The authors present a state‑of‑the‑art cosmological zoom‑in simulation aimed at testing whether major mergers in galaxy groups can generate the enigmatic odd radio circles (ORCs) recently discovered in deep radio surveys. Using the OpenGadget3 code, they model a halo that grows to a final virial mass of M₍₂₀₀₎ ≈ 1.8 × 10¹³ M⊙, a mass range appropriate for the host groups of observed ORCs. The simulation resolves dark matter particles with 4 × 10⁵ M⊙ and gas particles with 6 × 10⁵ M⊙, employing a modern smoothed‑particle magnetohydrodynamics (SPMHD) scheme, a Tree‑PM gravity solver, and a hyperbolic divergence‑cleaning method to keep ∇·B under control.

A key novelty is the on‑the‑fly treatment of cosmic‑ray (CR) electrons via the CRESCENDO module, which solves the Fokker‑Planck equation for each SPH element at every timestep. The only source term for CR electrons is diffusive shock acceleration (DSA); the acceleration efficiency depends on shock Mach number and obliquity as supplied by an embedded shock finder. Radiative cooling, star formation, and AGN feedback are deliberately omitted, so the plasma evolves purely under gravity, hydrodynamics, and magnetic forces.

During the simulation the halo experiences several minor accretion events, but a particularly violent major merger at redshift z ≈ 0.5 (mass ratio ≈ 1:2) dominates the subsequent evolution. The impact drives a network of strong shocks that propagate through the circum‑galactic medium (CGM). The shocks expand quasi‑spherically, reaching the virial radius (≈ 200 kpc) within ∼300 Myr. When the observer’s line of sight is perpendicular to the merger axis, the projected CR electron energy density forms a nearly perfect ring, reproducing the edge‑brightened morphology of ORCs.

Synthetic X‑ray maps (0.5–2 keV) show surface brightnesses of order 10⁻¹⁴ erg s⁻¹ cm⁻² arcmin⁻², consistent with observed groups of similar mass. Radio mock‑observations at 1.4 GHz reveal a ring with a total luminosity L₁.₄ ≈ 10³⁰ erg s⁻¹ Hz⁻¹, which is 1–2 dex lower than the luminosities measured for well‑studied ORCs such as “Physalis”. The simulated polarization fraction is 30–40 %, considerably higher than the 10–20 % typically reported. These discrepancies point to the limitation of a pure‑DSA electron budget.

The authors argue that fossil CR electron populations left over from prior AGN activity or supernova‑driven winds could be re‑accelerated by the merger‑driven shocks, boosting the radio power and reducing the polarization by providing a more tangled magnetic field. They also note that magnetic field topology, turbulence, and shock obliquity variations could further modify the observable properties.

In summary, the work demonstrates that major mergers in ∼10¹³ M⊙ galaxy groups naturally produce expanding shock fronts that, when viewed from the right angle, appear as large, edge‑brightened radio circles. The simulated X‑ray characteristics match observations, confirming that the underlying halo is realistic. However, to fully reproduce the observed radio luminosities and modest polarization levels, additional non‑thermal electron sources and more complex magneto‑hydrodynamic processes must be incorporated. The study thus provides a compelling proof‑of‑concept for the merger‑shock origin of ORCs while highlighting the next steps needed for quantitative agreement with the growing observational sample.