Cosmic dance of the ‘Space Clover’

April 30, 2024

Odd radio circles (ORC), a recently identified new class of extended faint radio sources, have captivated the curiosity of astronomers worldwide. A groundbreaking discovery by a team led by the Max Planck Institute for Extraterrestrial Physics unveils the first detection of diffuse X-ray gas in the vicinity of the Cloverleaf ORC. Leveraging the advanced capabilities of the XMM-Newton telescope and the complementary multi-wavelength observations, the team unveiled the origin of the ORC as a cosmic dance of two galaxy groups. 

Recent advancements in radio astronomy have led to the discovery of faint radio sources called Odd Radio Circles (ORCs), a new category of extragalactic sources distinguished by their unique morphology. Characterized by their distinctive features, ORCs exhibit a ring-like emission, prominently bright and clumpy at the edges, accompanied by irregular emission peaks within their centers. The Cloverleaf ORC is fairly close-by – its light had to travel ‘only’ about 600 million years to reach us (redshift 0.046). Initial observations suggested a potential association with a central elliptical galaxy.

A pioneering effort led by the Max Planck Institute for Extraterrestrial Physics (MPE) has now targeted the Cloverleaf ORC using the XMM-Newton observatory. The team's dedicated investigation has revealed unexpected findings. "Despite the marginal detection in the eROSITA X-ray data, we were eager to have a closer look at this intriguing system and were fortunate to secure the necessary telescope time," remarks Esra Bulbul, who is leading the group at the Max Planck Institute for Extraterrestrial Physics (MPE).  "To our surprise, the extended X-ray emission we detected exhibits a perpendicular orientation to the radio emission.”

The research team has identified two prominent peaks, each revealing intriguing characteristics. The southern X-ray peak, aligning precisely with the brightest radio emission, has an optical counterpart, an elliptical central galaxy. In contrast, the northeastern peak presents puzzling characteristics as it neither has an optical counterpart nor is associated with  radio emission. Further examination of the diffuse X-ray gas surrounding the Cloverleaf ORC suggests that the ORC is situated within a region filled with gas, permeating the space inside the small galaxy groups.

The team uncovered compelling evidence suggesting that the Cloverleaf Odd Radio Circle (ORC) system is in the midst of a merger. A closer examination of individual galaxies within the system, researchers have identified significant disturbances in their morphology, particularly towards the east. The study also highlights the presence of a high-velocity subgroup discernible at optical wavelengths. This, coupled with the disturbed morphology of the X-ray emission from the intra-group medium, strongly indicates ongoing merger activity within the Cloverleaf ORC system. Significant offsets between galaxies and X-ray gas are commonly observed in mergers of large galaxy clusters but rare within galaxy groups. Xiaoyuan Zhang, a postdoctoral researcher at MPE and the paper's second author, emphasizes the importance of deeper X-ray observations for tracing the thermal history of the intra-group medium and gaining deeper insights into the merging scenario. "If we want to trace the thermal history of the intra-group medium, which would shed light on the merging scenario, we need deeper X-ray observations in the future,” comments Zhang.

With only eight ORCs identified thus far, understanding their origin requires explaining their elusive nature. The leading theory on the rarity of ORCs stems from the narrow range of geometric factors necessary for their observation within the context of such mergers. A captivating hypothesis regarding the observed powerful radio signal suggests that the supermassive black holes residing within the ORC underwent periods of intense activity in the past. The accelerated electrons by this activity may be reaccelerated by the ongoing merging event, contributing to the observed powerful radio signal. “While the merger scenario naturally explains the characteristics of the Cloverleaf ORC, it's important to exercise caution, as this represents only one system," advises Esra Bulbul. "By conducting comprehensive observations across wide wavelength range, we aim to unravel their origins of other ORCs as well."


This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No 101002585)

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