ORC J0356-4216: Double-Ring Odd Radio Circle

• ORC J0356-4216 is an odd radio circle characterized by a rare symmetric double-ring structure detected solely in radio continuum surveys.
• Its photometric redshift (z≈0.494) and calculated physical size of ~668 kpc confirm it as a giant extragalactic radio feature with uniform spectral indices.
• Coherent polarisation and tangential magnetic field measurements, alongside equipartition field strengths, support a relic AGN lobe origin scenario.

ORC J0356-4216 is a distinct example within the class of Odd Radio Circles (ORCs), a recently discovered population of extended radio sources characterized by their ring-like morphologies and an absence of clearly associated counterparts at other wavelengths. Detected exclusively in radio continuum surveys, ORC J0356-4216 is notable for its rare symmetric double-ring structure. Detailing its properties and physical interpretation provides key insights into the diverse origins and evolutionary pathways of extragalactic radio features (Taziaux et al., 5 Sep 2025).

1. Redshift Determination and Host Identification

The redshift of ORC J0356-4216 is measured via photometric methods applied to its likely host galaxy, WISEA J035609.67−421603.5. Multi-band spectral energy distribution (SED) fitting, combining DESI Legacy Survey optical/near-infrared imaging with WISE photometry, yields a photometric redshift:

• $z = 0.494 \pm 0.068$

No spectroscopically derived redshift is currently available; thus, this photometric estimate, together with its associated formal uncertainty, is adopted for subsequent distance-dependent analyses. The host shows properties indicative of a massive system, bearing relevance for ORC origin models (Taziaux et al., 5 Sep 2025).

2. Spatial Extent and Physical Scale

The structure subtends an angular diameter of approximately $2'$ (arcminutes) on the sky. Employing a standard cosmology ($H_0 = 70$ km s⁻¹ Mpc⁻¹, $\Omega_m = 0.3$, $\Omega_\Lambda = 0.7$), the angular diameter distance at $z \approx 0.494$ is $D_A \approx 1.15$ Gpc. The corresponding physical size is computed via the standard formula:

$$L = \theta~(\mathrm{rad}) \times D_A~(\mathrm{kpc})$$

For $\theta = 2'$, this results in:

$$L \approx (2' \times \frac{\pi}{180\times60}) \times (1.15\times10^6~\mathrm{kpc}) \approx 668~\mathrm{kpc}$$

This establishes ORC J0356-4216 as a giant radio structure on group or cluster scales. No internal filaments, spokes, or substructure are evident within either the total intensity or polarised intensity maps (Taziaux et al., 5 Sep 2025).

3. Radio Continuum Properties and Spectral Index

ASKAP EMU data at $2'$0 MHz and MeerKAT L-band data at $2'$1 GHz provide measurements of integrated flux densities:

• $2'$2 mJy (total, both rings)
• $2'$3 mJy (total, both rings)

The spectral index $2'$4 (where $2'$5) is derived for each ring using these flux densities, supplemented by six-subband MeerKAT data:

• Ring 1: $2'$6
• Ring 2: $2'$7

The spectral index distribution across both rings is steep and spatially uniform, with no indication of gradients or localised spectral breaks ($2'$8). The absence of spectral substructure implies an energetically relaxed system, lacking ongoing acceleration sites such as bright hotspots or internal shocks (Taziaux et al., 5 Sep 2025).

4. Polarisation and Faraday Rotation

Polarisation imaging reveals a high degree of linear polarisation throughout much of the structure:

• Brighter (left) side: $2'$9
• Northern section: $H_0 = 70$0

Rotation measure synthesis over $H_0 = 70$1 MHz uncovers a coherent polarised signal, with negligible frequency-dependent depolarisation. The evolution of the polarisation angle $H_0 = 70$2 is well-described by:

$H_0 = 70$3

Measured rotation measures, foreground-corrected using the all-sky RM grid of Hutschenreuter et al. (2022), span $H_0 = 70$4 to $H_0 = 70$5 rad m⁻², with the mean Galactic foreground at $H_0 = 70$6 rad m⁻². Magnetic field vectors inferred from polarisation angle measurements are roughly tangential to the ring shells. These characteristics are broadly consistent with non-thermal synchrotron emission from large-scale magnetised plasma (Taziaux et al., 5 Sep 2025).

5. Equipartition Magnetic Field Strength and Radiative Losses

Minimum energy (equipartition) magnetic field values are estimated using the formalism of Beck & Krause (2005), with a proton-to-electron energy ratio $H_0 = 70$7, filling factor $H_0 = 70$8, and adopted shell thickness $H_0 = 70$9 kpc. Using a rest-frame frequency near $\Omega_m = 0.3$0 GHz, the results are:

Ring	$\Omega_m = 0.3$1 (μG)
1	$\Omega_m = 0.3$2
2	$\Omega_m = 0.3$3

A key implication is that the equipartition field strength is well below the threshold for dominant synchrotron losses at this redshift:

$\Omega_m = 0.3$4

This suggests that energy losses among the relativistic electron population are governed primarily by inverse-Compton scattering off the cosmic microwave background, as opposed to synchrotron radiation (Taziaux et al., 5 Sep 2025).

6. Morphology, Physical Interpretation, and Origin Scenarios

The double-ring structure of ORC J0356-4216 exhibits high symmetry, limb-brightening, and an absence of central jets, filaments, or hotspot substructure. Two principal models are discussed for its origin:

1. Large-Scale Shock from Starburst or Outflow: An explosive starburst or merger-driven outflow drives a spherical (or bipolar) shock through the circumgalactic medium, accelerating electrons at the expanding shell. Morphologically, this predicts limb-brightened, tangentially polarised structures. The host galaxy's high star formation rate ($\Omega_m = 0.3$5), typical of (U)LIRGs, is compatible with intense starburst activity and associated wind shocks.
2. Relic AGN Lobes Seen End-On: Jet activity from a prior episode of AGN feedback inflates twin plasma lobes. Spectral aging and subsequent expansion create steep, coherent ring-like features, with the observed symmetry emerging from projection effects. The lack of ongoing core activity, the high-mass ($\Omega_m = 0.3$6), red elliptical host, and the uniform spectral and polarisation properties support this scenario.

A plausible implication is that, while both scenarios can account for the steep spectral indices and polarisation, the extreme symmetry, coherence, and absence of interior complexity particularly favour an interpretation as relic AGN lobes viewed nearly along their axis. A starburst-driven shock origin remains possible, but is considered less strongly supported by the observed data (Taziaux et al., 5 Sep 2025).