RAD J131346.9+500320

RAD J131346.9+500320 (RAD J131346) is an odd radio circle (ORC) located in the constellation Canes Venatici approximately 7.7 billion light-years from Earth. It consists of two intersecting rings, each spanning 300,000 light-years, surrounded by an even larger radio cloud extending nearly 3 million light-years. It is the most distant and powerful ORC yet observed, and is the first ORC identified through citizen science collaboration.

RAD J131346.9+500320

RAD J131346.9+500320 is an example of an 'odd radio circle' (ORC). It is the most distant and powerful ORC yet found.
Object type	Odd radio circle
Observation data (Epoch J2000)
Constellation	Canes Venatici
Right ascension	13h 13m 51s
Declination	+50° 04′ 00″
Distance	7.7 billion light years

Discovery and observational details

Citizen science breakthrough

RAD J131346 was discovered on June 11, 2024, during an online training session of the RAD@home Astronomy Collaboratory, a pioneering citizen science initiative based in Mumbai, India. The discovery emerged through visual inspection of low-frequency continuum maps from the LOFAR Two-metre Sky Survey (LoTSS) DR2, when participants identified two intersecting ring-like structures centered on a compact radio core.

The RAD@home Collaboratory represents a novel approach to astronomical research, training science-educated citizens to analyze multi-wavelength astronomical data and identify rare cosmic phenomena that automated algorithms might miss. Founded in 2013 by Dr. Ananda Hota, this zero-funding, zero-infrastructure platform has encouraged astronomical research by enabling volunteers to participate in genuine scientific discoveries.

LOFAR telescope observations

The LOFAR core near Exloo, Netherlands.

The detection was made possible through the Low Frequency Array (LOFAR), the world's largest and most sensitive radio telescope operating at low frequencies between 10 and 240 MHz. LOFAR consists of 52 antenna stations distributed across eight European countries, creating a pan-European interferometer with unparalleled sensitivity and angular resolution at these frequencies.

LOFAR's innovative design utilizes thousands of simple antennas without moving parts, with signals digitally combined in software to create radio images. This revolutionary approach provides more than two orders of magnitude better sensitivity than previous telescopes at these frequencies, making it ideally suited for detecting faint, extended radio structures like ORCs.

Physical characteristics and structure

Ring system properties

RAD J131346 exhibits a remarkable double-ring morphology, with each ring measuring approximately 300 kiloparsecs (approximately 978,000 light-years) in diameter. The entire structure extends over 800 kiloparsecs (2.6 million light-years), embedded within diffuse emission that rivals the size of giant radio galaxies.

The twin rings display mild brightness enhancements at their intersection points, suggesting complex interaction between the overlapping structures. This configuration represents only the second known example of an ORC with intersecting rings, making it exceptionally rare among the handful of confirmed ORCs discovered to date.

Spectral analysis and power

Detailed spectral analysis reveals steep radio spectra with spectral indices of α₅₄¹⁴⁴ = 1.22 ± 0.15 and α₁₄₄¹⁴⁰⁰ = 1.20 ± 0.10. These consistently steep values across a wide frequency range support interpretation of the emission as aged synchrotron plasma, characteristic of relic emission rather than ongoing jet activity.

The integrated radio luminosity reaches 2.27 × 10²⁶ W Hz⁻¹ at 144 MHz, making it nearly two orders of magnitude more powerful than other known ORCs, which typically exhibit luminosities in the range 10²³–10²⁴ W Hz⁻¹. This extraordinary power, combined with its high redshift of z ≈ 0.94, establishes RAD J131346 as both the most distant and most powerful ORC identified to date.

Host galaxy and environment

Central galaxy properties

The radio emission originates from a faint optical galaxy (SDSS J131346.92+500319.3) with a photometric redshift of z = 0.937 ± 0.045. The central compact radio core exhibits a flat spectrum with spectral index ≈ -0.3, typical of active radio galaxy cores, and contributes 1.75 ± 0.18 mJy to the total flux density of 43.2 ± 4.1 mJy.

Galaxy cluster environment

The host galaxy resides within a galaxy group or poor cluster at redshift z ≈ 0.9, containing at least 15 galaxies with similar redshifts. This cluster environment, with a mass of approximately 10¹⁴ solar masses, provides crucial environmental context for understanding ORC formation mechanisms.

The presence of multiple galaxies with concordant redshifts within the ORC structure suggests that environmental density gradients and possible jet-galaxy interactions play central roles in shaping these ring morphologies. All three objects discovered in this study—including RAD J131346—are found in galaxy clusters of similar mass, highlighting the importance of cluster environments in ORC formation.

Formation mechanisms and theoretical models

Relic synchrotron origin

The steep radio spectrum and morphological characteristics strongly support a relic synchrotron origin for RAD J131346. ORCs are best understood as fossil radio shells that have been re-energized by external or internal processes, such as large-scale shocks induced by galaxy mergers, black hole mergers, or powerful superwinds.

Superwind model

Recent theoretical work proposes that the rings may be linked to superwind outflows from spiral host radio galaxies. If a bipolar superwind from the spiral host initiates after radio lobes have reached a remnant phase, twin radio rings can form and expand to ORC dimensions over hundreds of millions of years.

This model draws parallels to smaller-scale analogues like NGC 3079, where a polarized radio ring sits within a wind-blown radio bubble. Scaling up such processes under permissive environmental conditions could produce ORC-sized structures, particularly when involving "Speca-like" radio galaxies hosted by optically red disk galaxies capable of multiple episodes of growth.

Black hole feedback mechanisms

The formation of ORCs likely involves complex feedback processes between supermassive black holes and their host galaxies. Supermassive black holes can drive powerful jets and winds that interact with surrounding gas, creating shock waves and bubble-like structures.

Recent studies demonstrate that black hole jets can rapidly change direction within timescales of just one million years, potentially creating complex radio structures through multiple episodes of activity. These feedback mechanisms play crucial roles in regulating star formation and galaxy evolution, with radio observations providing unique insights into these processes.

Scientific significance and implications

Understanding cosmic evolution

RAD J131346 provides unprecedented insight into cosmic phenomena occurring when the universe was only half its current age. The discovery demonstrates that powerful radio structures were already present in the early universe, offering constraints on models of galaxy and black hole co-evolution.

The detection of such a distant and powerful ORC suggests that these phenomena may have been more common in the early universe, when galaxy formation and black hole growth were at their peak. This has important implications for understanding how supermassive black holes influenced their host galaxies through feedback processes during cosmic history.

Methodological advances

The discovery showcases the continued importance of human pattern recognition in identifying rare cosmic phenomena that escape automated detection algorithms. The RAD@home approach demonstrates how citizen science can complement professional research, particularly in the era of big data astronomy.

The success of LOFAR in detecting this distant ORC highlights the revolutionary capabilities of low-frequency radio astronomy. As more sensitive telescopes like the Square Kilometre Array come online, astronomers expect to discover many more ORCs, providing larger samples for statistical studies of these enigmatic objects.

Future research directions

The discovery of RAD J131346 opens new avenues for investigating ORC formation and evolution. Future observations with higher resolution and sensitivity will help determine whether the double-ring structure represents two distinct shells or a complex three-dimensional morphology viewed at an intermediate inclination.

Polarimetric observations will provide crucial insights into magnetic field structures within the rings, while X-ray observations may reveal associated hot gas that could illuminate the underlying formation mechanisms. The expected discovery of more ORCs will enable statistical studies of their environments, host galaxies, and physical properties.