W – T – F…
That’s what radio astronomer Anna Kapinska labelled the image
of an enormous, mysterious blob she first spotted in 2019.
Billions of lightyears from Earth, in the realms of deep space,
it appeared like a ghostly ring of smoke.
A glowing oddity of radio waves that didn’t appear to match up
with any known supernova remnant or galactic interaction.
What in the world - in the universe, could this be?
It may have been unusual, but this object was not alone.
Named “Odd Radio Circles”, or ORCs,
the objects we have found seem to encompass everything we struggle to explain in the cosmos.
Five ORCs have been confirmed since Kapinska’s first finding,
and several more blobs may fit the profile.
What cosmic force is so powerful
that it could sculpt a ring of energy a million light-years wide,
so vast it dwarfs our Milky Way by a factor of 10?
Are these radio rings the relics of galactic rebirth,
or the dying echoes of something we have yet to comprehend?
And why haven’t we found more of them?
I'm Alex McColgan, and you're watching Astrum.
Join me today as we attempt to unravel one of modern astronomy’s greatest mysteries,
and reveal the four leading hypotheses that could unlock
the origin of these objects so odd, they defy even the universe’s standards.
This mystery first began as a result of modern technology.
Around the turn of the new millennium, technological advancements
in radio astronomy drastically increased the number of known radio sources
from just a few hundred thousand, to a couple million.
For decades, nobody was able to significantly increase that number… that is, until recently.
Radio astronomy was due for another upgrade, and Australia’s national science agency,
the Commonwealth Scientific and Industrial Research Organisation, oversaw the launch of the
revolutionary Australian Square Kilometre Array Pathfinder telescope, or ASKAP – one of several
new radio telescopes coming online at the time. With 36 radio dishes that are each 12 metres
across, what makes the Western Australia radio telescope special is its phased array feed,
which allows it to image huge areas of the sky at once – unlike the previous generation
of radio telescopes. ASKAP’s field of view is an impressive 30 square degrees – the equivalent to
the surface area of about 150 moons as seen from Earth. The phased array feed allows astronomers
to ignore radio signals from things like satellites, which can otherwise create
blind spots for radio telescope receivers. With this new generation of telescopes,
radio astronomers are able to detect even faint, diffuse objects.
Western Sydney University professor, Ray Norris of CSIRO - later leading the original ORC discovery
- predicted that the Australian telescope’s Evolutionary Map of the Universe project, or EMU,
would be part of an extraordinary increase in our knowledge of radio sources across the universe.
The project is named after the “Emu in the sky”, a constellation of dark nebulae, rather than stars,
in the Milky Way galaxy, which has been recognised in aboriginal astronomy for generations.
All of the radio sources discovered up to that point, by every radio telescope in the entire
history of radio astronomy, amounted to about 2.5 million sources. And the EMU project alone
was expected to increase that total to 70 million radio sources. Keep in mind, that only represents
the discoveries that were expected to come from just one of the new radio astronomy telescopes.
Norris was certain that all of this new data would deliver some major surprises.
And it did – much sooner than anyone expected. The EMU project looked at large areas of sky
that other telescopes had not imaged yet, and collected vast catalogues of data – so
much that scientists expected they would need the help of machine learning to sift through it all.
But it was in the process of browsing through some of the project’s preliminary images that Kapinska,
the radio astronomer who discovered ORCs, would first notice the oddly shaped radio emissions,
using her old-fashioned human eyes. To me, that makes this discovery all the
more special. Even with vast amounts of data and machine learning tools, this is
an example of how human curiosity will always be an integral part of the scientific process.
While browsing these early EMU images, Kapinska started to notice some strange shapes that she
couldn’t identify as any known space object. On an image of one of these odd shapes, a circle of
radio emissions like a giant smoke ring floating in distant space, she wrote the label, “WTF”,
encapsulating just how baffling this was. Kapinska had never seen anything like it.
After sharing her strange discovery with colleagues, a team of radio astronomers
including Norris, and Emil Lenc, began sifting through the images, searching for more of these
circular radio blobs. Lenc found a second blob just a few days after the initial discovery,
and the team eventually coined the name Odd Radio Circles. At first, they weren’t even
sure if these were real objects, or just some kind of artifact or error from the telescope.
But they were able to use other telescopes to locate the ORCs, confirming that these odd
radio circles were in fact real. The question that followed was of course, “what are these things?”
Luckily, we’ve been able to learn a bit more about ORCs since Kapinska first noticed one.
This is an image of ORC 1 from the EMU Pilot Survey, a blurry grayscale blob. But luckily,
the EMU team also produced this second, colourised and enhanced image that helps to bring out less
obvious features, while still maintaining the original resolution. In the colour image,
ORC 1 now appears as a hazy green-blue blob. Because this hazy blob is only
actually visible in radio wavelengths, the colour in this image had to be added in so
that we can visualise it. It also helps us see more distinct features within the ORC structure.
At least a billion light-years away in deep space, initial estimates suggested that ORC
1 may have a diameter of around 1 million light-years, but more recently, observations
have indicated that it may actually be as wide as 2 million light-years across – that’s 20
times the width of our own galaxy. So we know how big ORCs can be,
and how distant they are… but what are they? At first, some thought these strange rings
might be the remnants of a powerful supernova. But the conditions were all wrong. For one thing,
we typically find supernova remnants near the Galactic Plane, and all of the ORCs
we’ve identified are too far away from it. Not only that, but it appears that some of these
ORCs might have galaxies in their centres, which is not something we see with supernova remnants.
What’s more, the scale is all wrong, too. Most supernova remnants are a few dozen light-years
across, but these ORC rings are far, far larger. Big enough to encompass entire
galaxies at their centre, the ORCs we’ve observed so far are hundreds of thousands,
up to 2 million light-years across. These are not supernova remnants – ORCs must be something else.
ORCs could also be something we are already familiar with – like the jets from a radio
galaxy – but the vast distance, or maybe an odd angle, might obscure our ability to identify them.
Since the initial discovery of ORC 1, astronomers have had the chance to observe it with additional
telescopes, revealing an object that appears to be at the centre of the circle that can be
seen in both visible and infrared light. Using the South African MeerKAT radio
telescope – an array of 64 interlinked receptors – Norris and his team observed ORC 1 for 10
hours to create an image of the circle. They overlaid that on optical data from the Dark
Energy Survey to reveal an elliptical galaxy in the centre of the orc. With this, they were able
to estimate the central galaxy’s distance from Earth, at around 5 billion light-years away.
Not only did they find this central elliptical galaxy,
but the high sensitivity image also revealed a complex inner structure of knots and arcs,
with additional galaxies found at these knots. If Norris and his team are correct and this
elliptical galaxy is indeed located near the centre of ORC 1, then it might have had
a hand in creating this radio circle. However, it’s also possible that the
galaxy’s location is a coincidence, as it may just be near the apparent centre
of the ORC, rather than the actual centre. Armed with this new image and information,
in their 2022 paper, Norris and his team considered three different origin
stories for the ORC, with the data not favouring one hypothesis over another.
The first possibility was that ORC 1’s spherical shell was the result of a cataclysmic event in the
elliptical host galaxy, like the merging of two supermassive black holes, for example. In this
scenario, a merger of two supermassive black holes would send a shock wave out in all directions,
accelerating electrons in the intergalactic medium. This could result in a bubble of radio
emissions, which could appear to us as a circle of radio emissions with a brightened edge, or ring.
If this were the case, we would expect the ORC to contain a supermassive black hole,
or an active galactic nucleus. And not only does ORC 1 show evidence of a radio-loud galactic
nucleus like we would expect if this hypothesis were correct, but so do two other observed ORCs,
lending further credibility to this hypothesis. The second possibility offered by Norris and his
team for ORC 1’s origin was that it could simply be radio jets from a radio-loud
Active Galactic Nucleus, or AGN, viewed end-on. Radio-loud AGNs are a type of galaxy that emits
large amounts of energy from its nucleus in the form of two plasma jets, as matter falls into a
supermassive black hole at their centre. And as I mentioned, we already know that AGNs are present
in many of the ORCs we have observed thus far. If we were to view this kind of system end-on,
where the two jets were superimposed, it could create the appearance of what
we see with ORC 1 – lobes of radio emissions superimposed, with a host galaxy at the centre.
However, we still need 3D simulations to help us work out whether or not these particular
AGNs are producing radio jets and if so, whether or not we are viewing them head-on.
The third and final hypothesis for an origin story could be that the radio circle of ORC 1 is caused
by what’s called a “starburst termination shock.” In this explanation, millions of stars would have
been born during a brief, intense period of star formation in the elliptical host galaxy.
During this time, new stars would have been born at a rate much higher than normal, and
would have lasted for just a few tens of millions of years – just a blink on the cosmic time scale.
These stars would emit solar wind that expands out into space over billions of years,
exciting electrons and creating a spherical radio emission bubble like what we see in ORC 1.
While the team modelled this possibility in more detail than the first two hypotheses for
the formation of ORC 1, showing mathematically how this scenario was plausible, Norris was clear
that the data did not favour this explanation over the others. So, while we do have several
promising ideas, the origin of ORC 1 is still as mysterious as the odd radio circle itself.
As each new ORC has been discovered, the only thing we seem to know for sure is that not all
ORCs are created equally. After all, the name Odd Radio Circles is meant as a descriptive
name – like an umbrella term for any odd radio circle that can’t be described by an
existing type of object. So, while all ORCs may include odd radio circles, each individual ORC
may have its own unique origin story. The discovery of ORC 2 and ORC 3, the
second and third Odd Radio Circles to be identified, didn’t do much to help solve
the mystery of how ORCs are created. In fact, they introduced even more questions.
Seen in this grayscale image, the two ORCs appear to be very close to each other,
nearly touching. They’re roughly the same size, but on the right side of the image,
ORC 2 is bright and clearly ring-shaped, while ORC 3, located to the left, is faint and appears
more like a disk. In fact, ORC 3 may not even be recognisable in the grayscale EMU image,
if it weren’t for the light grey speckles seeming to cluster together into a circular shape right
next to the more distinct ring shape of ORC 2. Now take a look at this colour image of ORC 2
and ORC 3: the diffuse radio emission has been enhanced in red, and overlaid on top of the
original radio pattern in green. You can see the obscured green colour behind the red glow,
where the ring-shape of ORC 2 is located, and to the left, you can see its faint sister
circle of ORC 3, only visible in diffuse red. The proximity of these ORCs would suggest they
are related somehow, but if that’s the case, why do they appear so different?
As much as I’d like to tell you that we’ve solved this curious case of ORC 2 and ORC 3
since their discovery, which was published in 2021, the nature of these sister ORCs
remains an unsolved mystery to this day. Before moving on to the theories for ORC
4’s origin, I’ve got to mention ORC 5, which was discovered in Australian Square
Kilometre Array Pathfinder data in 2021. While we don’t have any leading theories
for ORC 5 specifically, this ORC was found to have a central elliptical galaxy, like ORCs 1
and 4. And interestingly, ORC 5 was the only one found to have both this central galaxy,
and a nearby neighboring galaxy found at approximately the same distance from the
ORC ring, which suggests these two galaxies are likely interacting gravitationally.
While there may be some repeating characteristics, for every ORC we’ve confirmed so far, it seems
that each one has their own unique qualities. Of course, as we discover and confirm more ORCs, it
may be that this won’t be the case in the future. But with so few ORCs, and so little observational
evidence, it’s been difficult for scientists to nail down one theory for their origin.
And that brings us to ORC 4, dubbed the Cloverleaf ORC because of its irregular shape. Of course,
this turned out to be another unique case. First identified in archival data from the Giant
Meterwave Radio Telescope, the Cloverleaf ORC would end up being the most extensively observed
and imaged odd radio circle to date, having been observed in radio, x-ray, and visible wavelengths.
In this stunning image of the Cloverleaf ORC, wavelengths from several observations have
been combined, with radio emissions shown in red, optical wavelengths depicted in white to yellow,
and X-ray emissions in blue. This ORC resides some 600 million light-years from Earth, and stretches
more than 325,000 light-years across – a little more than three times the width of our galaxy.
In 2024, astronomers at the W. M. Keck Observatory on Maunakea, Hawaii detected
strong fluorescent light from oxygen atoms coming from the Cloverleaf ORC,
stretching across 130,000 light-years of the ORC’s massive structure. This Oxygen-2 nebula of heated,
compressed gas – also called “shocked” gas – seemed to have been created by a
fast-moving explosion, according to Alison Coil, astronomer and astrophysicist at University of
California San Diego, who led the research team that conducted this study of ORC 4.
Since the Cloverleaf ORC also has a central elliptical galaxy,
Coil stated that one possible explanation was that this ORC could have been formed by
a starburst termination shock event – one of the three possible hypotheses for the origin of ORC 1.
The team was able to calculate the age of the stars inside the galaxy in the Cloverleaf ORC,
and found that they are 6 billion years old. Using this and other data, they ran a simulation that
showed how a similar structure to the Cloverleaf ORC could have been created by a starburst event
that caused a high-mass outflow rate. Both the radio emission sphere, and this gas nebula sphere,
could have been blown out by the central galaxy by the shockwave of combined supernova explosions.
While this is an exciting possibility, especially because of its similarities with one of the
ORC 1 hypotheses, it’s not the only possibility. That same year, using the European Space Agency’s
XMM-Newton X-ray telescope, a team led by scientists at the Max Planck Institute for
Extraterrestrial Physics in Germany detected diffuse X-ray emissions for the first time in
an odd radio circle, coming from the Cloverleaf ORC. Based on their observations, the same team
that observed the X-ray emission, led by German astrophysicists Esra Bulbul and Xiaoyuan Zhang,
hypothesised that the Cloverleaf ORC may be linked to a cosmic dance between two groups of galaxies.
The X-ray observations suggest that the Cloverleaf ORC could be the result of a
messy merger of two galaxy groups that gravitated toward each other. This type of a galactic merger
could have created shock waves that would leave behind a gigantic bubble of radio emissions,
like what we’ve seen in observations of this ORC. However, a lot is still unknown,
and it’s unclear why this set of galactic groups would produce such an ORC, while other similar
galactic group mergers don’t seem to leave behind the same gigantic bubble of radio emissions.
As you can see, the mystery of how ORCs came to be is far from solved. In fact,
it’s still up for debate whether the Cloverleaf ORC is in fact an ORC at all, seeing as it is
more of a diffuse cloud and lacks the tell-tale bright edges of radio emissions. Norris points out
that similar cloverleaf-shaped radio clouds have been observed in other galaxy groups,
not all of which are associated with ORCs. The key to resolving which objects are part of the ORC
family, and which are not, is more data. Deeper radio and x-ray observations may be able to help
identify more complex structures and help differentiate between actual and near-ORCs.
Luckily, several other candidate ORCs have been identified, and they may offer
additional data for astronomers to study and compare to the already known cases.
Now instead of W – T – F, the side note on these strange radio emissions has a proper name: ORCs.
But even though the jury is still out on how ORCs came to be, we’re constantly gaining new
insights from the likes of Project EMU and ASKAP. While studying EMU data for a nearby dwarf galaxy
called the Small Magellanic Cloud, astronomers spotted the ghostly remains of two supernova
remnants that were previously unknown to science, adding to our catalogue of celestial objects.
In addition to those, another nearby supernova remnant was found using the EMU survey in 2023,
and turned out to be one of the closest supernova remnants to our Earth, at just 4,200 light-years
away. This supernova remnant known as this, was found to be about 130-light-years across,
and is thought to be just 13,000 years old. Not only has EMU aided in the discovery of
previously unknown supernova remains, but it is also paving the way for using Machine Learning
and the power of citizen science to identify and catalogue some 4 million radio sources,
which could include ORCs, as part of a project called The Radio Galaxy Zoo: EMU, or RGZ EMU.
As of 2025, the project has had more than 2,000 volunteers and has helped to classify more than
92,000 sources. But the coolest part? You can join in and be part of this citizen science project
yourself! To learn more, visit zooniverse.org and search for the Radio Galaxy Zoo: EMU project.
As our tools continue to improve, I believe that astronomers are bound to discover more ORCS, and
even more objects we don’t know even exist yet. We may learn not just how these radio rings formed,
but we could also learn more about the life cycles of galaxies, the effects of starburst events,
and how we may be able to trace gas from ancient events far outside their galaxy of origin.
For now, though, the mystery of the origin of these Odd Radio Circles
persists – although with several promising hypotheses. And to unravel this mystery,
astronomers will have to conduct cosmic archaeology on a scale almost too big to fathom.
Solving the mystery of ORCs isn’t just about finding them in the sky —
it’s about analysing mountains of data, spotting subtle patterns,
and testing ideas against the evidence.
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