Astronomers have recently unraveled the origins of a giant, enigmatic radio structure nicknamed the “Cloverleaf.” Using the X-ray satellite XMM-Newton, a team led by Esra Bulbul, an astrophysicist at the Max Planck Institute for Extraterrestrial Physics, has discovered that the Cloverleaf formed from the violent collision of two galaxy groups.
These enormous, circular radio features, collectively known as ORCs (odd radio circles), were only discovered in 2021 with the help of advanced radio surveys. Each ORC is vast, capable of engulfing entire galaxies, and their faint radio emissions puzzled scientists.
“The energy required to generate such widespread radio waves is immense,” Bulbul explained. “While simulations can recreate their shapes, they fall short in explaining their intensity. We don’t have a clear picture of how ORCs form.”
Enter X-ray astronomy. Bulbul and her colleague Xiaoyuan Zhang examined data from eROSITA, a German-Russian X-ray telescope. They spotted faint X-ray emissions potentially originating from the Cloverleaf, based on a mere 7 minutes of observation.
This tantalizing hint prompted them to secure more observation time with XMM-Newton, a powerful X-ray observatory co-developed by the European Space Agency (ESA) with contributions from NASA.
“We received about five and a half hours of observation,” Bulbul said. “When the data arrived late one evening in November, I sent it to Xiaoyuan. The next morning, he burst into my office saying, ‘Detection!’ I was ecstatic!”
Zhang elaborated, “We were fortunate. eROSITA observations revealed several plausible X-ray point sources near the ORC, but not the extensive emission we found with XMM-Newton. The eROSITA sources likely weren’t from the Cloverleaf, but they were compelling enough to warrant further investigation.”
The X-ray emissions act like a cosmic fingerprint, tracing the distribution of gas within the colliding galaxy groups. By analyzing the disturbed gas, scientists were able to determine that the Cloverleaf arose from the merger of two distinct galaxy groups. The temperature of the gas emissions also provided clues about the number of galaxies involved.
Galaxy mergers trigger a dramatic increase in gravitational pull due to the combined mass. This immense gravity causes surrounding gas to fall inwards, heating it significantly. The hotter the gas, the greater the system’s total mass.
The X-ray spectrum revealed a gas temperature of roughly 15 million degrees Fahrenheit (8-9 million degrees Celsius). “This measurement suggests that the Cloverleaf ORC is home to roughly a dozen galaxies that have gravitationally bound together,” Zhang explained, “which aligns with observations in deep visible light images.”
The team proposes that the merger generated shock waves that accelerated particles, leading to the observed radio emission.
“Galaxy group mergers are frequent occurrences,” commented Kim Weaver, the NASA project scientist for XMM-Newton at Goddard Space Flight Center. “However, the source of the accelerated particles remains unclear. One intriguing possibility is that the resident supermassive black holes underwent periods of extreme activity in the past. Relic electrons from those ancient events could have been reignited by this recent merger.”
While galaxy group mergers are commonplace, ORCs are exceptionally rare. The exact mechanism by which these interactions produce such powerful radio emissions is still under investigation.
“Mergers are the cornerstone of structure formation in the universe,” Bulbul said. “But there’s something unique about this system that amplifies the radio emission. We don’t yet understand what that is. More extensive data, from both radio and X-ray telescopes, is crucial.”
While the team has unraveled the nature of the Cloverleaf ORC, their work has opened a new chapter of inquiry. Further, in-depth studies of the Cloverleaf hold the promise of unraveling the mysteries of these enigmatic structures.
“These intricate interactions encompass a vast array of scientific disciplines,” Weaver concluded. “It’s like a miniature universe, offering a glimpse into nearly every aspect of the cosmos.”