Odd Radio Circles (ORCs): Formation & Properties

• Odd Radio Circles are an emerging class of ring-shaped radio sources featuring steep synchrotron emission and limb-brightened morphologies.
• They are detected across multiple frequencies with facilities like ASKAP, MeerKAT, and LOFAR, revealing sizes from 100 to 800 kpc and associations with diverse host galaxies.
• Current studies propose formation mechanisms including merger-driven shocks, AGN outbursts, and virial shocks, though no single model yet explains all observed properties.

Odd Radio Circles (ORCs) are an emerging class of faint, large, predominantly edge-brightened circular radio sources at high Galactic latitude, typically several hundred kiloparsecs in diameter, displaying steep-spectrum synchrotron emission with no clear optical or X-ray counterparts on similar spatial scales. Since their first discovery in the Evolutionary Map of the Universe Pilot Survey with the Australian Square Kilometre Array Pathfinder (ASKAP), multiple ORCs have now been identified in wide-area interferometric radio surveys utilizing ASKAP, MeerKAT, LOFAR, and other facilities, as well as in low-frequency citizen-science search campaigns. The origin of ORCs remains under active investigation, with proposed scenarios including merger-driven shocks, AGN outbursts, remnant radio lobes, virial shocks, supernovae remnants in the intragroup medium, and re-energized fossil cosmic ray populations.

1. Discovery and Observational Properties

Early detections of ORCs occurred in the ASKAP EMU pilot survey as morphologically distinct, limb-brightened radio rings of ∼1′ angular diameter, corresponding to 300–500 kpc at inferred redshifts of z ≈ 0.2–0.6 when associated with central elliptical galaxies (Norris et al., 2020, Koribalski et al., 2021, Rupke et al., 2023). These structures maintain their edge-brightened, symmetric morphology in both higher frequency MeerKAT and lower frequency LOFAR observations, confirming that they are not instrumental artefacts or limited to a single frequency regime (Omar, 2022, Norris et al., 2022). Spectral indices are typically steep (α ≲ –1), and polarimetric analysis indicates strong tangential alignment of the magnetic field vectors along the ring (fractional polarizations up to ∼30%), consistent with shock-compressed synchrotron emission (Norris et al., 2022, Taziaux et al., 5 Sep 2025). Sizes span from ∼100 kpc for the smallest currently detected with high-sensitivity deep surveys (e.g., MIGHTEE/MeerKAT; (Norris et al., 26 Nov 2024)) up to ∼800 kpc for systems at z ≳ 0.9 (Hota et al., 2 Oct 2025). Some ORCs exhibit complex morphologies, including multi-ringed and double-lobed structures (Taziaux et al., 5 Sep 2025, Hota et al., 2 Oct 2025), and a subset shows faint central radio cores coincident with massive, red, quiescent host galaxies (Koribalski et al., 2021, Rupke et al., 2023).

2. Multiwavelength Counterparts and Host Galaxy Associations

High-resolution optical imaging and spectroscopy have established that many ORCs with identifiable central components are centered on massive (M* ≈ 10¹¹ M⊙), red, quiescent elliptical galaxies with little or no ongoing star formation, faint emission-line nebulae (LINER-type), and radio-quiet or low-luminosity AGN activity (Rupke et al., 2023). In several cases, extended stellar features such as shells and tidal tails surrounding the host suggest recent mergers or strong interactions (Norris et al., 26 Nov 2024). For a minority of ORCs, particularly those lacking a dominant central galaxy, association is inferred with a nearby moderate-luminosity AGN in a disk galaxy, suggesting diversity in triggering mechanisms (Rupke et al., 2023). Environmental studies reveal that ORCs tend to lie in galaxy overdensities or possess close companions, implying that denser environments, and possibly enhanced ambient density or magnetic fields, play a role in shock formation and electron acceleration (Norris et al., 2021, Bulbul et al., 14 Mar 2024).

3. Physical Mechanisms and Theoretical Models

A variety of formation scenarios have been explored. No existing class of radio source (supernova remnants, planetary nebulae, traditional radio galaxy lobes, star-forming rings, Wolf–Rayet bubbles, gravitational lensing artefacts) accounts for the full set of observed ORC properties (Norris et al., 2020):

• Merger-driven shocks: Cosmological MHD zoom-in simulations show that major mergers in ∼10¹³ M⊙ galaxy groups can drive strong shocks into the circumgalactic medium, yielding shell- or ring-shaped synchrotron radio emission matching the spatial scale and morphology of ORCs for certain viewing angles (Ivleva et al., 1 Aug 2025, Koribalski et al., 2023, Bulbul et al., 14 Mar 2024).
• AGN jet-inflated bubbles and fossil re-acceleration: Three-dimensional cosmic-ray MHD and relativistic hydrodynamics simulations demonstrate that large, edge-brightened radio rings consistent with ORCs arise when CR proton–dominated AGN jets inflate bubbles in low pressure environments and then undergo shock re-energization, producing secondary electrons (ε_sync ∝ e_cr·ρ·B²) and sharp edge-brightening at the interface (Lin et al., 16 Jan 2024, Shabala et al., 15 Feb 2024). The "phoenix" model posits that faded remnant radio lobes from prior episodes of radio-loud AGN activity can be revived by merger or cluster shocks, with the re-energized electrons generating ORC-like morphologies, both circular and elliptical, dependent on the shock's orientation and incidence angle (Shabala et al., 15 Feb 2024).
• Virial shocks: ORCs may arise as the synchrotron signature of virial shocks at the outer boundary of massive halos (M_vir ≥ 10¹³ M⊙), accelerated as gas accretes onto the halo. In this model, the observed shell width is explained by cosmic-ray electron diffusion (D_cr ∼ 10³⁰ cm² s⁻¹), and low-mass halos lack sufficiently strong shocks to produce detectable radio emission (Yamasaki et al., 2023).
• Supernova remnants in the intragroup medium (IGrM): Although SNRs can produce similar limb-brightened structures in low-density environments, population synthesis and radio light curve modeling reveal that only a small subset (≈8%) of ORCs can be attributed to SNRs in the IGrM, with the bulk requiring larger energies and scales (Omar, 2022, Sarbadhicary et al., 2022).
• Tidal disruption events (TDEs): The cumulative effect of multiple TDEs in post-starburst galaxies can inject sufficient energy (E_tot ∼ 10⁵⁵–10⁵⁹ erg over ∼10⁸ yr) to drive shocks capable of generating ∼100–500 kpc shells observable as ORCs (Omar, 2022).

4. Population Studies, Selection Effects, and Evolutionary Connections

Early samples were strongly biased toward high surface brightness, large-diameter systems detected in moderate-depth radio surveys (Norris et al., 2020, Koribalski et al., 2021). More recent, deeper, higher-resolution surveys (e.g., MeerKAT/MIGHTEE) find smaller (D ∼ 100 kpc) and fainter rings, implying that the ORC flux density distribution follows that of the general extragalactic radio population, with shallow surveys preferentially detecting only the upper end of the size/luminosity function (Norris et al., 26 Nov 2024, Gupta et al., 10 Jun 2025). Machine learning–based detection pipelines and citizen-science visual searches have established that ORCs are rare (typical sky density ≲ 1 per 50 deg²) but that selection sensitivity is a limiting factor. Related classes such as GLAREs (Galaxies with Large-scale Ambient Radio Emission) and SRRGs (Starburst Radio Ring Galaxies) may lie on an evolutionary continuum with ORCs, with GLAREs representing precursors or faded descendants and SRRGs marking starburst-driven ring systems with different radio-luminosity–host correlations (Gupta et al., 10 Jun 2025).

A summary table of recently reported ORCs, their salient properties, and host associations is as follows:

ORC/Reference	Diameter (kpc)	Redshift (z)	Host Type / Environment
ORC 1–5 (Norris et al., 2020)	300–500	0.27–0.55	Red elliptical, quiescent, LINER
J0219–0505 (Norris et al., 26 Nov 2024)	114	0.196	Elliptical, tidal features
J0356–4216 (Taziaux et al., 5 Sep 2025)	668	0.494	Old stellar pop., double-ring
J131346.9+500320 (Hota et al., 2 Oct 2025)	800	∼0.94	Most distant, intersecting rings
Cloverleaf (Bulbul et al., 14 Mar 2024)	230×160	0.046	Group merger, diffuse X-rays

5. Multi-frequency and Multiwavelength Diagnostics

The steep observed spectral indices (typically α ≲ –1) require sustained electron acceleration or re-acceleration. Polarimetric and Faraday rotation mapping show tangential magnetic field alignment along the ring, supporting shock compression and ordered fields. In several cases, equipartition magnetic fields inferred from radio data are on the order of 1–2 μG, below the inverse-Compton threshold B_IC ≈ 3.25(1+z)² μG, reinforcing the role of synchrotron/IC cooling and favoring advanced electron ages (Norris et al., 2022, Taziaux et al., 5 Sep 2025). The absence of extended X-ray emission in most ORCs (with the notable exception of the Cloverleaf system) supports models wherein the ambient medium is hot but low in density (∼10⁻⁴–10⁻³ cm⁻³) and in group-scale halos rather than rich clusters (Bulbul et al., 14 Mar 2024). A key LaTeX formula used for shell width in virial shock models is

$$\delta \approx \frac{2\sqrt{4 D_{\rm cr} t_{\rm IC}}}{r_{\rm sh}}$$

connecting the shell width δ, the CR diffusion coefficient D_cr, the IC cooling time t_IC, and the shock radius r_sh (Yamasaki et al., 2023).

6. Environmental and Dynamical Context

ORCs are preferentially found in environments where merging, tidal interactions, or overdensities are present. Recent XMM–Newton mapping of the low-z Cloverleaf ORC resolves multiphase X-ray gas over ∼200×150 kpc, indicative of an intragroup medium in a merging system (Bulbul et al., 14 Mar 2024). The presence of off-centered diffuse X-ray peaks, high-velocity subgroups, and galaxies with disturbed morphologies (tidal features, gas stripping signatures) is typical, supporting dynamical triggers for large-scale shocks. In some systems, both radio and X-ray emission are spatially offset, explicable as the result of lag between collisionless galaxies (and fossil CR plasma) and decelerated collisional gas in a merger (Bulbul et al., 14 Mar 2024, Ivleva et al., 1 Aug 2025).

7. Outstanding Questions and Prospects

No single model accounts for the full diversity of observed ORC properties, including their size distribution, morphological symmetry, host galaxy variations, environment, and spectral polarization characteristics. Major open questions include the relative role of AGN feedback, mergers, fossil CR reacceleration, SNRs in the intragroup context, and the requirement for group-scale, rather than cluster- or field-scale, conditions. Discrepancies between simulated and observed radio luminosities and polarization fractions imply that contributions from both fresh shock acceleration (DSA) and fossil energetic electron populations are likely necessary (Ivleva et al., 1 Aug 2025).

Future deep, wide-area radio sky surveys (e.g., SKA, expanded MeerKAT, LOFAR) combined with systematic multiwavelength follow-up (optical IFU, X-ray, mid-IR) and advances in machine learning–based source identification are expected to increase the known ORC sample significantly and enable tests of physical models through population statistics, environmental correlations, and detailed energetics. The measurement of CR diffusion coefficients in the CGM (from shell-thickness analyses), direct detection of X-ray shocked gas, and refined polarimetric diagnostics will further constrain the origin and evolution of these rare and enigmatic radio structures.