An ORC named 'Cloverleaf' is captured in a new light

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In recent years, astronomers have stumbled upon a peculiar phenomenon in the vast expanse of the universe: enormous circular radio features surrounding some galaxies. Structures like the featured named Cloverleaf, collectively known as odd radio circles (ORCs), have puzzled scientists since their discovery in 2021.

Thanks to advancements in technology, radio surveys have become sensitive enough to detect these faint signals, leading to the identification of eight ORCs scattered randomly beyond our galaxy.

Hunting the Cloverleaf ORC

Esra Bulbul, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and her postdoctoral researcher Xiaoyuan Zhang, have been at the forefront of unraveling the mystery of ORCs.

Their study focused on the Cloverleaf ORC, which has provided valuable insights into the formation and nature of these enigmatic structures.

"This is the first time anyone has seen X-ray emission associated with an ORC," said Bulbul. "It was the missing key to unlock the secret of the Cloverleaf's formation."

X-ray vision unlocks ORC secrets

Bulbul and Zhang began their investigation by examining data from eROSITA, an orbiting German/Russian X-ray telescope.

They noticed some X-ray emission that seemed to originate from the Cloverleaf, based on a mere 7 minutes of observation time.

This finding prompted them to assemble a larger team and secure additional telescope time with XMM-Newton, an ESA mission with NASA contributions.

"We were allotted about five-and-a-half hours, and the data came in late one evening in November," Bulbul recounted. "I forwarded it to Xiaoyuan, and he came into my office the next morning and said, 'Detection,' and I just started cheering!"

Assembling the Cloverleaf ORC puzzle pieces

The X-ray emission observed by the team traces the distribution of gas within the group of galaxies, much like police tape around a crime scene.

By analyzing how the gas has been disturbed, scientists determined that the galaxies embedded in the Cloverleaf are actually members of two separate groups that drew close enough together to merge.

"We really got lucky," Zhang said. "We saw several plausible X-ray point sources close to the ORC in eROSITA observations, but not the expanded emission we saw with XMM-Newton. It turns out the eROSITA sources couldn't have been from the Cloverleaf, but it was compelling enough to get us to take a closer look."

Shock waves from a galactic merger

The team proposes that the merger of the two galaxy groups produced shock waves that accelerated particles, resulting in the observed radio emission.

The temperature of the X-ray emission, around 15 million degrees Fahrenheit (8-9 million degrees Celsius), suggests that the Cloverleaf ORC is hosted by approximately a dozen galaxies that have gravitated together.

"Galaxies interact and coalesce all the time," said Kim Weaver, the NASA project scientist for XMM-Newton at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in the study.

"But 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," Weaver concluded.

Elusive nature of ORCs holds their mystery

While the team has solved the mystery of the nature of the Cloverleaf ORC, their findings have also raised additional questions. Galaxy group mergers are common, but ORCs are extremely rare, and it remains unclear how these interactions can produce such strong radio emissions.

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

Bulbul and her team plan to study the Cloverleaf in more detail to unravel these remaining mysteries. "Mergers make up the backbone of structure formation, but there's something special in this system that rockets the radio emission," she said. "We can't tell right now what it is, so we need more and deeper data from both radio and X-ray telescopes."

Window into the cosmos

The study of ORCs and galaxy mergers offers a unique opportunity to explore a wide range of cosmic phenomena.

As Weaver aptly put it, "We stand to learn a lot from more thorough observations because these interactions take in all kinds of science. You've pretty much got everything that we deal with in the cosmos put together in this little package. It's like a mini universe."

Cloverleaf, ORCs, and the future of radio astronomy

In summary, the discovery of the Cloverleaf ORC and the groundbreaking research conducted by Esra Bulbul, Xiaoyuan Zhang, and their team have opened a new chapter in our understanding of the universe.

Their findings highlight the complex interactions between galaxies and the formation of these enigmatic structures. As astronomers continue to explore the mysteries of ORCs and galaxy mergers, we eagerly anticipate the new insights and discoveries that will emerge from their tireless efforts.

The study of the Cloverleaf serves as a testament to the power of collaboration, advanced technology, and the indomitable human spirit in our quest to unravel the secrets of the cosmos.

The full study was published in the journal Astronomy and Astrophysics.

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