Deep polarimetry study reveals double ring ORC-like structures

TL;DR

This study presents a radio spectropolarimetric analysis of ORC J0356--4216, a newly discovered double-ring Odd Radio Circle. Using MeerKAT L-band and ASKAP/EMU data, the authors characterize the morphology, spectra, and magnetic fields, finding two symmetric rings with diameters of ~2 arcminutes and redshift z ≈ 0.494, corresponding to a physical scale of ~668 kpc. The rings have steep spectra (indices ~-1.1 to -1.2) and equipartition magnetic fields around 1.6–1.8 μG, with 20–30% polarization and magnetic fields tangential to the rings. The polarization and symmetry patterns, along with the host’s properties, favor an AGN relic or jet-driven outflow origin over a starburst-driven shock scenario, though both remain plausible; the work also provides a framework for estimating the space density of ORCs and highlights the need for deeper, high-resolution surveys for population studies.

Abstract

New observations with the current generation of advanced radio interferometers, such as ASKAP and MeerKAT, have led to the discovery of new classes of extended radio sources of unknown origin, including the so-called Odd Radio Circles (ORCs). These phenomena are detected exclusively in the radio continuum, with no clear counterparts at other wavelengths, making their physical nature and origin a subject of ongoing investigation. To better understand these objects, we study their radio continuum emission, spectral characteristics, and magnetic field properties. In this work, we present a radio spectropolarimetry analysis of a newly discovered ORC (ORC J0356-4216) that exhibits a rare double-ring morphology. We use data from the MeerKAT L-band and from the ASKAP Evolutionary Map of the Universe (EMU) at 943 MHz. ORC J0356-4216 shows a symmetric double-ring structure with a diameter of approximately 2 arcminutes, corresponding to a physical size of 668 kpc based on the redshift ($0.494 \pm 0.068$) of its apparent host galaxy WISEA J035609.67-421603.5. The radio spectra of both rings are steep, with spectral indices of $-1.18 \pm 0.03$ and $-1.12 \pm 0.05$, and show no significant substructure. Equipartition magnetic field strengths (assuming K0 = 1) are estimated to be 1.82 microGauss and 1.65 microGauss for the respective rings. The degree of polarisation across the object ranges between 20-30%, consistent with a non-thermal synchrotron origin. The morphology and polarisation are broadly consistent with large-scale shocks driven by powerful starburst outflows. However, the high degree of symmetry, the coherent double-ring structure, and the absence of internal substructure are features commonly associated with relic AGN lobes, making this scenario particularly compatible with the observed characteristics.

Figures (7)

• Figure 1: Left: Color-composite image from the DESI Legacy Imaging Surveys of ORC J0356–4216 with overlaid MeerKAT L-band radio emission contours starting at $3\,\sigma$ and increasing by a factor of 2 at a central frequency of 1.28 GHz with a noise level $\sigma=3\,\upmu$Jy/beam. The scale in the top left corner is calculated using the redshift of the host galaxy WISEA J035609.67--421603.5. Right: MeerKAT L-band radio emission with overlaid contours starting at $3\,\sigma$ and increasing by a factor of 2 at a central frequency of 1.28 GHz with a noise level $\sigma=3\,\upmu$Jy/beam. The arrows are pointing to the host galaxy WISEA J035609.67--421603.5 and the bright radio source SUMSS J035607--421626. The beam of $8.57\arcsec \times 7.65\arcsec$ is shown in the bottom left corner.
• Figure 2: WISE color–color diagram showing in the background the locations of classes of objects Wright_2010, while showing the different detected ORCs. The WISE W1, W2 and W3 bands are $3.4\,\upmu$m, $4.6\,\upmu$m and $12\,\upmu$m, respectively. Arrows indicate upper limits.
• Figure 3: Overview of the relevant radio continuum polarimetry parameters of ORC J0356–4216. Panel a): Total intensity emission of MeerKAT data at central frequency of 1.28 GHz and bandwidth of 0.758 GHz with overlaid contours starting at $3\sigma$ and increasing by factor of two ($\sigma = 3\,\upmu$Jy/beam). The beam of $8.57\arcsec \times 7.65\arcsec$ appears in the lower left corner. Panel b): Total intensity emission of EMU data at central frequency of 943 MHz and bandwidth of 0.288 MHz with overlaid contours starting at $3\sigma$ and increasing by a factor of two ($\sigma = 20\,\upmu$Jy/beam). A circular beam of $15\arcsec$ appears in the lower left corner. Panel c): Two-point total spectral index map between the EMU data of 943 MHz and MeerKAT data of 1280 MHz with overlaid contours starting at $3\sigma$ and increasing by a factor of two ($\sigma = 5.5\,\upmu$Jy/beam). The circular beam of $15\arcsec$ appears in the lower left corner. Panel d): WISE $3.4\,\upmu$m map with overlaid contours of the MeerKAT 1.28 GHz emission, starting at $3\sigma$ and increasing by factor of two ($\sigma = 3\,\upmu$Jy/beam). The beam of $8.57\arcsec \times 7.65\arcsec$ appears in the lower left corner. Panel e): Polarised intensity emission of MeerKAT data at central frequency of 1.28 GHz. The contours drawn make use of the signal-to-noise map, starting then at a factor 7 and increase by $\sqrt{2}$. The circular beam of $22\arcsec$ appears in the lower left corner. Panel f): Fractional polarisation of MeerKAT data at central frequency of 1.28 GHz with the same overlaid contours from the polarised intensity map. The contours drawn make use of the signal-to-noise map, starting then at a factor 7 and increase by $\sqrt{2}$. The circular beam of $22\arcsec$ appears in the lower left corner. Panel g): Polarised intensity emission of MeerKAT data at central frequency of 1.28 GHz with overlaid total intensity emission at 1.28 GHz contours starting at $3\sigma$ and increasing by a factor of two ($\sigma = 30\,\upmu$Jy/beam). The magnetic field orientations are shown in black and scale with the polarised intensity. The circular beam of $22\arcsec$ appears in the lower left corner. Panel h): Distribution of foreground corrected rotation measure of MeerKAT data at central frequency of 1.28 GHz with overlaid total intensity emission at 1.28 GHz contours starting at $3\sigma$ and increasing by a factor of two ($\sigma = 30\,\upmu$Jy/beam). The circular beam of $22\arcsec$ appears in the lower left corner.
• Figure 4: Left panel: Fitted spectra of the different components of ORC J0356--4216, including ring 1, ring 2, and the host galaxy. The plotted data correspond to the EMU data and the six sub-bands of the MeerKAT observations. We assumed 10 % integrated flux density uncertainty for the EMU data and 4 % for the MeerKAT data. Right panel: MeerKAT 1.28 GHz background map showing the different components of the galaxy.
• Figure 5: Left panel: Photometric redshift distribution of all the sources around ORC J0356--4216 taken from Legacy Survey DR9 photo-z Zhou_2021. All galaxies, below a photometric redshift of 0.35 are shown in light-blue and all galaxies above redshift 0.65 are displayed in pink. Middle panel: Kernel density estimate map of galaxy positions in the same field, showing the relative projected source density, based on the source information from the Legacy Survey DR9. Right panel: Spatial distribution of stellar masses for galaxies in the field, derived from WISE $W1$ photometry and photometric redshifts, using the stellar mass calibration from Jarrett_2023.