Astronomers have stumbled upon a mysterious cosmic phenomenon called Odd Radio Circles, or ORCs.
These strange circular radio features, discovered around certain galaxies, have puzzled scientists ever since their first discovery in 2021.
It was previously unknown how these huge circles formed.
Now, a new observation of one particularly intriguing ORC, known as the Cloverleaf, is shedding light on its origin. Researchers postulate that it formed through the dramatic clash of massive groups of galaxies.
An X-ray satellite allowed the scientists to see this mysterious structure in a new way. The team suggests that the merging triggered supersonic shock waves, which rapidly accelerated particles within the Cloverleaf. These accelerated particles, thus, created the X-ray emissions observed in this ORC.
“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,” said Esra Bulbul, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, who led the study.
Extremely hot surrounding gas
Scientists used X-ray emissions to trace the distribution of gas within the Cloverleaf structure, revealing that it was generated by the merging of two independent groups of galaxies.
When galaxies merge, their total mass increases, causing the surrounding gas to heat and generate X-rays. The X-ray spectrum emission indicates that the gas is around 15 million degrees Fahrenheit (roughly between 8 and 9 million degrees Celsius.)
“That measurement let us deduce that the Cloverleaf ORC is hosted by around a dozen galaxies that have gravitated together, which agrees with what we see in deep visible light images,” said Xiaoyuan Zhang, a postdoctoral researcher.
ORCs are hard to find
Interestingly, the scientists suggest that the acceleration of particles from this ORC is likely associated with a black hole.
“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,” said Kim Weaver, the NASA project scientist for XMM-Newton.
ORCs are extremely difficult to detect because of their faint signals. So far, just eight of these formations have been identified in random locations beyond our galaxy. And they are massive enough to “envelop an entire galaxy — sometimes several.”
“The power needed to produce such an expansive radio emission is very strong. Some simulations can reproduce their shapes but not their intensity. No simulations explain how to create ORCs,” said Bulbul in the press release.
Cloverleaf’s powerful emission was first spotted using data from eROSITA (Extended Roentgen Survey with an Imaging Telescope Array), an orbiting German/Russian X-ray observatory. The follow-up observations were made using ESA’s XMM-Newton.
ORCs are rare, and their strong emissions remain a mystery. The team further plans to acquire deeper insights from both of these telescopes.
The findings were reported in the journal Astronomy and Astrophysics Letters.
