An ORC Named “Cloverleaf” Leads To a Big Discovery

Among all the mysteries of the universe, a recently discovered phenomenon has emerged as a particularly intriguing one. In 2021, an important discovery was made by astronomers: they found huge circular radio features of unknown origin around some galaxies. These structures, designated as Odd Radio Circles, are exceptionally large enigmatic astronomical objects that, at radio wavelengths, manifest as circular and brighter along their edges.

Now, new X-ray observations, particularly that of an object designated as “the Cloverleaf,” are providing new insights into these cosmic enigmas, suggesting that collisions between groups of galaxies may be a potential origin.

“This is the first time anyone has seen X-ray emission associated with an ORC, it was the missing key to unlock the secret of the Cloverleaf’s formation.”

— Esra Bulbul, astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany

Enigmatic objects

Prior to 2021, the scientific community was unaware of the existence of odd radio circles (ORCs). However, the development of more sensitive radio-sounding technology has facilitated the detection of eight of these enigmatic structures, which have been observed beyond our galaxy. What makes ORCs particularly intriguing is their massive size, which is capable of encompassing entire galaxies or even multiple galaxies within their circular boundaries.

The emission of intense radio signals by ORCs has presented a puzzling challenge to astronomers. While some simulations were able to reproduce the shapes of these structures, none were able to explain the intensity of their radio emissions. This discrepancy has fueled curiosity and prompted further investigation into the origins and nature of ORCs.

“The power needed to produce such an expansive radio emission is very strong,” Bulbul said. “Some simulations can reproduce their shapes but not their intensity. No simulations explain how to create ORCs.”

Now, Dr. Esra Bulbul and postdoctoral researcher Xiaoyuan Zhang, utilizing the eROSITA X-ray telescope and subsequently the XMM-Newton in a joint ESA-NASA mission, have identified X-ray emissions associated with ORCs.

The discovery, particularly the Cloverleaf, represents a significant advancement in astrophysical research. The discovery not only provided crucial insights into the formation and nature of these enigmatic structures but also offered scientists a chance to probe everything from supersonic shock waves to black hole behavior, opening new avenues of exploration in understanding the cosmic phenomena that shape our universe. 

The results of these recent observations were published in Astronomy and Astrophysics Letters on April 30.

The source of emissions

The analysis of the initial data from an ORC in X-rays revealed that the gas within the group of galaxies had been significantly disturbed. This data enabled scientists to determine that the galaxies embedded in the Cloverleaf are members of different groups that have merged in the past.

The emission’s X-ray spectrum provided information about the emission’s temperature, with the result indicating that it is between 8 and 9 million degrees Celsius. This measurement indicates that the Cloverleaf ORC is hosted by approximately a dozen galaxies that have gravitated together. 

The team proposed that the merger produced shock waves that accelerated particles to create radio emissions. However, it is still unclear how these interactions can produce such strong radio emissions, in addition to the fact that galaxy group mergers are common in the universe but that a group of galaxies rarely causes an ORC. 

“The source of the accelerated particles is unclear. One fascinating idea for the powerful radio signal is that the resident supermassive black holes went through episodes of extreme activity in the past, and relic electrons from that ancient activity were reaccelerated by this merging event.”

— Kim Weaver, the NASA project scientist for XMM-Newton at NASA’s Goddard Space Flight Center in Greenbelt, Maryland

In conclusion, this significant discovery has resolved a significant mystery of the universe, it has also opened additional questions that the team is motivated to answer with new deeper data from both radio and X-ray telescopes.