Cosmological zoom-in simulation of odd radio circles as merger-driven shocks in galaxy groups

¹ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München Scheinerstr. 1 81679 München, Germany
² Department of Astronomy and Astrophysics, The University of Chicago, William Eckhart Research Center 5640 S. Ellis Ave. Chicago IL 60637, USA
³ Max-Planck-Institut für Astrophysik Karl-Scharzschild-Str. 1 85748 Garching, Germany
⁴ Australia Telescope National Facility, CSIRO, Space and Astronomy P.O. Box 76 Epping NSW 1710, Australia
⁵ Western Sydney University Locked Bag 1797 Penrith South DC NSW 2751, Australia

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 1 August 2025
Accepted: 9 December 2025

Abstract

Context. A new class of distinct radio objects, commonly referred to as odd radio circles (ORCs), has been recently discovered. The origin of these features remains unclear because their peculiar properties challenge our current understanding of astrophysical sources for diffuse radio emission.

Aims. We test the feasibility and limits of major mergers in galaxy groups as a possible formation channel for ORCs.

Methods. By modelling the assembly of a massive galaxy group with a final virial mass of M₂₀₀ ∼ 10¹³ M_⊙ in a magnetohydrodynamic zoom-in simulation with on-the-fly cosmic ray treatment, we derive the X-ray and radio properties of the system self-consistently and compare them to observations.

Results. We show that the X-ray properties of the simulated system agree with characteristics of observed galaxy groups in the relevant 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 triggers a series of strong shocks in the circumgalactic medium, which in unison form a ring if the line of sight is perpendicular to the merger axis. The shock is rapidly expands radially and quickly reaches the virial radius of the halo. This formation channel thus readily explains the morphology and large extent of ORCs. However, the inferred radio luminosity of these features is lower than that of observed counterparts, while the degree of polarisation seems systematically over-predicted by the simulation.

Conclusions. Fossil cosmic ray populations from active galactic nuclei 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.