X-ray spacecraft reveals odd 'Cloverleaf' radio circle in new light (image)

A splotchy pink and blue blob in the center of the image. In the background, space.
This multiwavelength image of the Cloverleaf ORC (odd radio circle) combines visible light observations from the DESI (Dark Energy Spectroscopic Instrument) Legacy Survey in white and yellow, X-rays from XMM-Newton in blue, and radio from ASKAP (the Australian Square Kilometer Array Pathfinder) in red. (Image credit: X. Zhang and M. Kluge (MPE), B. Koribalski (CSIRO))

XMM-Newton, a European Space Agency and NASA-operated space telescope, has imaged a vast cosmic "Cloverleaf" to uncover its mysterious origins.

The Cloverleaf is an example of an "odd radio circle," or ORC. These objects are strange bubbles of radio light that are so huge they can be thousands of times the width of the Milky Way, thus encompassing an entire galaxy — sometimes, many.

ORCs were discovered in 2019 when the Australian Square Kilometer Array Pathfinder (ASKAP) picked up on the aptly named ORC-1. Since then, radio surveys of the cosmos have become sensitive enough to detect a further seven ORCs, one of which is the Cloverleaf, the subject of XMM-Newton's observations.

The power needed to create such a structure is immense, leading astronomers to ponder about what events could be violent enough to create ORCs. Thanks to the observations XMM-Newton, researchers think the Cloverleaf's creation event was a collision between two groups of galaxies.

Related: The mystery of vast 'cosmic ORCs' — odd radio circles that encompass entire galaxies — may be solved

"This is the first time anyone has seen X-ray emission associated with an ORC," Esra Bulbul, leader of the team behind this investigation and an astrophysicist at the Max Planck Institute for Extraterrestrial Physics, said in statement. "It was the missing key to unlock the secret of the Cloverleaf's formation."

How to X-ray an ORC

A multitude of computer simulations have attempted to reconstruct the births of ORCs and have been able to recreate the shapes of these strange formations. Yet, none have recreated the intensity of the expansive radio emissions that define an ORC.

Bulbul realized, however, that ORCs hadn't been studied in X-ray light before. The scientist reasoned such a study may constitute the missing piece of the puzzle. Together with Max Plank Institute postdoc Xiaoyuan Zhang, she set scanning data from the Extended Roentgen Survey with an Imaging Telescope Array (eROSITA) in search of such ORC-related emissions. 

The duo found data of an X-ray emission that seemed to be linked to the Cloverleaf ORC gathered during just 7 minutes of eROSITA observing time. Though this was a tiny amount of data, however, it was enough to prompt Bulbul to gather a larger team and get five and a half hours of telescope time with XMM-Newton.

"We really got lucky," Zhang said in the statement. "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."

This image of the first ORC (odd radio circle) ever discovered, aptly dubbed ORC-1. (Image credit: Bärbel Koribalski / ASKAP)

A clash between galaxies

X-ray emission from the Cloverleaf seen by XMM-Newton shows the distrubution of gas within a group of galaxies embedded in the ORC. This is sort of like a chalk outline at a crime scene. 

By observing how this gas has been disturbed, the team could see that galaxies within the Cloverleaf were actually once part of two separate groups that drew together, collided and merged.

The X-ray emission also revealed the temperature of gas in the region, placing it at around around 15 million degrees Fahrenheit (8.3 million degrees Celsius). The greater the masses of galaxies involved in galactic pile-ups like this one, the greater the gravitational influence of the merger and the faster the gas gets dragged in. All of that, in turn, increases the temperature of infalling gas, meaning the temperature of this material can give scientists a clue as to how many galaxies were involved in the merger. You'd just need to work backward.

"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," Zhang said.

As for the radio emissions from the ORC, the team suggests these were generated by particles accelerated by shockwaves that rippled out from the galaxies as they were crashing together.

Artist's impression of an odd radio circle exploding from a central galaxy and expanding past other galaxies. (Image credit: Sam Moorefield; Data: CSIRO, HST (HUDF), ESA, NASA; Image: J. English (U. Manitoba), EMU, MeerKAT, DES, CTIO)

One issue with the team's proposal is that galaxy group mergers are common, but ORCs, on the other hand, are rare. That means it isn't yet certain how this particular galaxy group smashing together created the Cloverleaf, while similar events haven't birthed such ORCs.

"Galaxies interact and coalesce all the time," Kim Weaver, the NASA project scientist for XMM-Newton at NASA's Goddard Space Flight Center who was not involved in the study, said in the statement. "But the source of the accelerated particles is unclear." 

She added that another possibility for the powerful radio signal may involve the the supermassive black holes that sit at the heart of each of the colliding galaxies. These black holes may have gone through episodes of feeding and extreme activity in the past. Relic electrons from that ancient activity could remain and may have been reaccelerated by galaxy group merger producing intense radio emissions.

The mystery of the Cloverleaf may have been solved, but the findings raise more questions about these vast radio emissions. The team will continue to study this ORC to settle those queries. 

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

The team's research was published on Tuesday (April 30) in the journal Astronomy and Astrophysics Letters.

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Robert Lea
Senior Writer

Robert Lea is a science journalist in the U.K. whose articles have been published in Physics World, New Scientist, Astronomy Magazine, All About Space, Newsweek and ZME Science. He also writes about science communication for Elsevier and the European Journal of Physics. Rob holds a bachelor of science degree in physics and astronomy from the U.K.’s Open University. Follow him on Twitter @sciencef1rst.

  • Unclear Engineer
    The thing that springs to my mind when I hear that ORCs are the "sometimes" result of merging galaxies is that maybe they come from the ones which have their central supermassive black holes merge.

    We predict that occasionally happens, but do we have models for the emissions when it does?

    I wonder what such an event would do to life on any planets within the galaxy were those emissions started.
  • billslugg
    SMBH's that have lots of stuff to feed on are quasars.
  • Unclear Engineer
    We see lots of quasars that are not in the middle of ORCs, so I doubt that rapid feeding SMBHs are the source of ORCs.

    But two SMBHs merging into one SMBH is a different situation. We think that must have happened many times because we don't know of any other way to build up billions of solar masses into a single black hole.

    Something that can send out shock waves still visible when they have grown larger than galaxy clusters must have been more powerful than any other event that I can think of.
  • billslugg
    I don't know anything about ORC's, all I know is merging SMBH's can form quasars. That's what you asked.
  • Unclear Engineer
    billslugg said:
    I don't know anything about ORC's, all I know is merging SMBH's can form quasars. That's what you asked.
    ??? This article is about ORCs.

    I didn't mention quasars, I only said that MERGING supermassive black holes seem to be a likely candidate for the creation of ORCs.

    You FIRST said that quasars are caused by SMBH s feeding on lots of stuff, which is a likely situation during the merging of GALAXIES. I agree with that.

    But, the MERGING of 2 SMBHs is not the same thing as a SMBH feeding on lot of stuff. It probably would not last as long as we have observed quasars to last, for instance. And, it would likely create some monster gravitational waves.

    On the other hand, when 2 galaxies merge and their SMBHs approach each other and orbit close to each other, I would not be surprised if that does cause the SMBHs to feed rapidly on the stuff surrounding them, so that may be the situation for quasars, rather than a single SMBH somehow feeding rapidly. Two SMBHs orbiting each other closely would make the usual orbits for other stuff around each very unstable, probably enahancing the feeding rates..
  • billslugg
    Right, you asked what happened when SMBH's merge and I gave you the answer, provided there is infeeding material - quasar. If it happened in isolation, not so much.
  • Unclear Engineer
    You still seem to be missing my point. The actual merger of 2 SMBHs is not likely to be a long-term event. A quasar is probably NOT the actual merging event, but maybe the prelude to one, where 2 SMBHs are in close orbits BEFORE merging. That would fit the observation that there are a lot more quasars than ORCs in our field of view.

    But, on the other hand, I am not clear on how far away we are currently able to detect ORCs. Maybe their population density is closer to the density of quasars.

    It would not surprise me if all galaxy mergers went through phases that include their central SMBHs creating a quasar, and then merging and emitting an ORC shock wave, and then a quiescent period where the material near the now-merged SMBH is depleted by the shock from the merger.

    Which brings me back to my wondering what all that energy emission in the center of a galaxy would do to the environments of planets orbiting stars in their galaxies. Could life survive? If so, then maybe there is some life form that is extremely old out there somewhere. IF not, then that would say something about the prospects for life forms older than the last merger time for any particular galaxy.
  • billslugg
    Yes, I agree, the event of the merging of SMBH's is not what a quasar is. A quasar is simply a very large SMBH, formed at one time by the merging of smaller ones, that sits in a spot where a lot of matter can inflow. It is the flowing of matter into a very large SMBH that causes all the excitement. I don't know anything about ORCs.