15 mins read 11 May 2021

“What Is That?” – Astronomers Find A New Odd Radio Circle In Space

Australian astronomers have stumbled across another mysterious object known as an ‘Odd Radio Circle’ - detected so far only at radio frequencies - bringing the total population now to five ever discovered. But as yet, we don’t know what they are or what could be causing them - which leads to observers working together and exploring possible formation mechanisms, resulting in excellent science.

Image of ORC-2 and ORC-3, based on ASKAP radio continuum images at 944 MHz from the EMU Pilot Survey, with additional enhancements to emphasise the diffuse emission. Credit: Norris et al. 2021/CSIRO.

“We don’t have an explanation as yet”.

It’s not often you hear those words coming from scientists, but when you do – it means something really exciting is happening.

It’s even more exciting when it’s related to astronomy and astrophysics – as it could mean something big has been resolved (like a potential discovery of the elusive and exotic dark matter) or something new is being observed that requires explanation (like the mysterious fast radio bursts).

It’s also the kind of thing that astronomers and astrophysicists dream would happen to them along the way in their careers. A discovery of something new, that could potentially create a paradigm shift in our understanding of the Universe, and ultimately, how, and why we got here.

The mystery, the chase, the thrill, before the vigorous moment of Eureka!

Originally, these moments occurred when looking at or photographing the night sky in detail and finding new features that weren’t there prior, or noticing the behaviour of objects and phenomena that seemed a little different from all models. But these days, this can also include looking through terabytes of archival data collected by powerful telescopes or even commissioning machine learning algorithms to do some of the heavy lifting in data search.

Now, an Australian-led team of astronomers is having one of these exact moments, after their recent discovery of an ORC (an Odd Radio Circle) which are large circular structures detected so far only at radio frequencies that are most likely extragalactic.  When the first one was discovered in 2019, astronomers designated them “WTF” so at least things have progressed on the naming front.

“Objects with circular or near-circular radio emission are not unusual, including some supernova remnants and the inner star-forming rings in spiral galaxies,” said lead author and Professor Bärbel Koribalski from CSIRO’s Australian Telescope National Facility (ATNF).

“So, initially, the hunt resulted in numerous detections of the latter: I found several beautiful radio rings which - after checking the optical images - turned out to be nearby galaxies seen face-on.” 

“Until one day in June 2020, I found an ORC, i.e. a circular radio source not so far detected at any other wavelength. It felt great to discover this odd source, which is very similar to ORCs 1 and 4, especially since this is the only one I have found despite looking through tons of ASKAP data,” she said. 

“The discovery kick-started a lot of detective work as well as gathering a small research team and resulted in the paper accepted for MNRAS Letters.”

This latest discovery was made using the Australian Square Kilometre Array Pathfinder (ASKAP) telescope, an instrument owned and operated by Australia’s national science agency, CSIRO, and now accounts for the fifth ORC ever found. The results have been accepted for publication in MNRAS Letters (Koribalski et al. 2021) and are available on arXiv.

Sharing in the excitement of this latest find was co-author, Professor Ray Norris who also led a prior paper that found similar structures using ASKAP.

“It was incredibly exciting - most of our research involves gradually dragging an answer out of a long process of analysis - every scientist dreams of just confronting an unexpected new phenomenon in front of them on their screen,” he said.  

“Now I spend hours just gazing at the ORCs on my screen hoping an answer to what they are will just suddenly pop into my head. But it never does!”

ASKAP radio continuum of ORC-5 overlaid onto an optical colour image created from the Dark Energy Survey (DES). DES image cutouts of the three radio-detected galaxies and their average photometric redshifts are shown on the right side (top: background galaxy, middle: central galaxy, bottom: south-eastern galaxy). Credit: Koribalski et al. 2021/CSIRO.

This particular case, known as ORC J0102-2450 has a diameter of roughly 70 arcseconds, which can be translated to approximately 1 million light-years across (to compare how big this is, the distance between the Sun and Neptune is just over four light hours).

Curiously, ORC J0102-2450 features a central elliptical galaxy at its core – a trait that two other ORCs also have. The galaxy’s light in several optical wavelength bands from the Dark Energy Survey provided an approximate distance, suggesting it resides about 3.7 billion light-years from us.

Astronomers have some ideas about what ORCs could be, but with such a small population (currently), the jury is still out and there’s more work to be done.

“We've found a handful in 300 square degrees of sky, and we expect them to be spread more or less uniformly across the sky, so that means there are probably about 500-1000 of them waiting to be found,” said Ray. 

Australian institutions that were involved in the study include CSIRO, Western Sydney University, and the University of Tasmania. They were joined by researchers from Universidad de Guanajuato in Mexico and the University of Minnesota in the United States.

The first three ORCs were discovered in the Evolutionary Map of the Universe (EMU) pilot survey. The full EMU survey will map the sky south of +30-degrees declination across the 800 – 1088 MHz frequency range and - no doubt - discover many more ORCs. The fifth ORC, named ORC J0102-2450, was also discovered in ASKAP data towards the starburst galaxy NGC 253.

The objective of the EMU survey is to take a census of radio sources across the sky using ASKAP, expecting to detect about 70 million radio sources, mostly distant galaxies showcased at different stages of their evolution, allowing scientists to draw models and ideas about how the first stars and galaxies are born. Naturally, when conducting a survey of this scale and nature – odd things (like mysterious, unexplained radio-frequency circles in space) are going to pop up. 

What we know so far about Odd Radio Circles

ASKAP radio continuum contours of ORC J0102–2450 overlaid onto a WISE RGB colour image. Credit: Koribalski et al. 2021/CSIRO.

The first few ORCs were found in September 2019 when Dr Anna Kapinska (who is now at the National Radio Astronomy Observatory in New Mexico, USA), and Dr Emil Lenc from CSIRO’s ATNF were looking at the data (with good old fashion eyeballs instead of machines) captured by ASKAP as part of the EMU Pilot survey.

In the original paper, Norris et al. (2021) reported that four ORCs had been detected – three in ASKAP’s data (ORC-1, ORC-2, ORC-3), and a fourth (ORC-4) in the archival data (from 2013) of the Giant Metrewave Radio Telescope (GMRT). Additionally, these objects were followed up with the CSIRO’s Australia Telescope Compact Array (ATCA) which observed ORC-1, ORC-2 and ORC-3 in the 1.1 GHz – 3.1 GHz range.

“You always hope that it’s going to uncover a brand-new phenomenon, rather than just being a known object viewed in a different way,” said Ray.

ORC-5 was found by Koribalski et al. (2021) in a deep ASKAP field towards the nearby Sculptor Galaxy (NGC 253) over nine observations, conducted between August 2019 – December 2020. Each of the fully calibrated radio continuum images totalled 10 hours of integration time, with the band centred on 943.5 MHz, achieved using the Phased Array Feed receivers to form 36 beams, arranged in the 6 x 6 configuration.

Across all ORCs discovered so far, including these latest findings with ORC-5, all objects have presented themselves in radio wavelengths only, and have so far not been detected in any other bands of the electromagnetic spectrum, including X-rays, infrared or optical frequencies. Because they are only found in radio frequencies and are so far not very common, scientists think that they may be giant radio galaxies seen end-on or possibly something new, like a giant explosion in the central galaxy creating a blastwave, the remnant of which is an ORC.

ASKAP radio continuum image of ORC 1 overlaid onto a DES 3-color composite image. Note the two bright galaxies near the letter ‘C’. Credit: Norris et al. 2021/CSIRO.

GMRT image (325 MHz) of ORC 4 captured overlaid onto an SDSS image. Credit: Norris et al. 2021/Venturi et al. 2017.

Interesting, three of these structures, ORC-1, ORC-4 and now ORC-5, all have an elliptical galaxy in their centre – something the team says can’t just be a coincidence and is likely going to play a major role in explaining these objects.

“There are now three ORCs with similar-looking massive elliptical galaxies right in the centre. This is very, very unlikely a coincidence,” said Bärbel. 

“So, I think these are the host galaxies and associated with the radio rings. The central galaxies in the three ORCs are at redshifts of 0.3 - 0.5, resulting in ring diameters of 300 - 500 kpc. This is large!”

All of the ORCs discovered so far also seem to be located well away from the Galactic Plane at high galactic latitudes, which also supports the notion that they reside at great distances, otherwise we would expect to see them along with the band where most of the stars in our galaxy reside. They’re also about 1 arc minute across in diameter which translates to about 3% the size of the full moon in our sky.

Given three of these objects have galaxies in the centres and taking into consideration that these galaxies are likely associated with the ORCs, then astronomers can utilise the redshift of these galaxies (from the Dark Energy Survey)  – with ORC-1 located at z ~ 0.55 (7.5 billion light-years), ORC-4 at z~ 0.39 (5.3 billion light-years), and now ORC-5 located at z ~ 0.27 (3.7 billion light-years).

By using this distance, and the measured diameter of these three objects, the team has suggested that ORCs are on average roughly 1-million light-years in diameter. Like ORC-1, ORC-2, and ORC-4, this latest finding of ORC-5 is a ring structure, whilst ORC-3 displays a uniform disc structure.

“There should be some at closer redshift. The fact that none were detected before we made our discoveries tells us that ORCs are rare, relatively small and of low brightness,” said Bärbel. 

“There is no doubt that ASKAP continuum surveys will detect many more in the next few months and years. High-resolution low-frequency radio surveys are also very likely to detect numerous ORCs,” she said. 

However, the structures wouldn’t be as easily detected if we were to try and find them around our own galaxy. 

“If one surrounded our own Milky Way it would be so large and diffuse that we would be completely unable to see it,” said Ray.

A few Possible Explanations ...

ASKAP image (10 hours integration time at 944 MHz) of the Fornax A (NGC 1316) Galaxy. The galaxy is located in the centre of the giant two radio lobes extending outwards. Credit: Emil Lenc/CSIRO.

Whilst the small population of ORCs has meant that no definite conclusions can yet be drawn about what they are, finding more of these objects has astronomers narrowing down on what they potentially can be and at least ruling out some things they can’t be.

“It is great to be in a situation where we can use all our knowledge, expertise and experience across frequencies and domains together with new ideas to investigate this new type of sources,” said Bärbel.

“It makes for great discussions, and everyone can participate and speculate, conduct follow-up observations, modelling, make predictions and check them out.”

Giant Galaxy Radio Lobes

In this latest paper, Prof. Bärbel Koribalski and her team suggest that one explanation about what ORCs could be are giant radio galaxy lobes, seen end-on. These structures would emerge when the central jet of the galaxy has switched off, but the shells of radio lobes continue to expand into the intergalactic medium. Studies involving hydrodynamical simulations that looked at the shape and the expansion of radio lobes from galaxies after its supermassive black hole driven jet had turned off seem to support this idea, with the simulations showing large bubbles that were viewed end on are resembling ORCs.

“The jury is still out. Giant radio lobes seen end-on or down the barrel must exist and may in some circumstances look like ORCs,” said Bärbel. “In that case, ORCs would just be a known type of astronomical source seen at a specific and rare viewing angle.”

But, like every good mystery in science, there’s a range of competing views and ideas about what they could be. 

“We all have our favourite models, and our arguments about them sometimes get quite heated,” said Ray. 

“My personal view is that it's unlikely to be radio lobes because known radio lobes are so irregular and the ORCs are so beautifully circular. I think it's much more likely to be a spherical shell from the central galaxy, either as the result of a humongous explosion in the centre, or else as the result of a shock wave from an outflow of gas, such as a starburst wind.” 

“On the other hand, it has to be said that none of our models really fit all the facts yet. I think we’re missing something important,” he said.

Binary SMBH merger + giant blast wave

Another idea being put forward is that ORCs are the observed spherical shock waves radio remnants of a giant blast wave, produced by the merger of two supermassive black holes in the central galaxy. The spherical shells made of radio emissions would appear as edge-brightened discs, similar to the structure observed in the ORCs.

Interacting galaxy

Galaxy interaction between the M81 galaxy and surrounding smaller galaxies. Visible light (left image) shows the galaxies each fairly distant from the other, but the right image captured the cold hydrogen gas between them as it is tidally dragged. Credit: NRAO/AUI/NSF.

Based on this latest discovery of ORC-5 and potentially ORC-1, these structures could be the result of interactions from nearby galaxies and the surrounding intergalactic medium, featuring narrow tails for the galaxies that are curving in such a way that they form a semi-ring structure.

“So far, only ORC 5 has a central galaxy AND a neighbour at a similar redshift within the ring. These two galaxies are likely to be gravitationally interacting, but none of the other ORCs - given our current knowledge - fit this scenario,” said Bärbel. 

“When observing the 21-cm emission of atomic neutral hydrogen (HI) in and around galaxies, rings are sometimes detected in interacting galaxy pairs, and many of these will also be detected by ASKAP as part of the WALLABY project.

What ORCs are not

Several different phenomena have already been ruled out as ORCs, so astronomers are starting to close in on their explanation, even through a process of elimination. This has included imaging artifacts, lens flaring or glitches – with these ideas being quashed when multiple telescopes confirmed their existence (there would be very little chance of all telescopes experiencing these same issues).

Given their circular radio structure, supernovae remnants were another idea considered, but these were also quickly ruled out as all ORCs found so far appear to be away from the Galactic Plane – which is where we expect supernova remnants to be located (in line with the higher density of stars in the region). And why would they all have a distant galaxy in their centre?

Similar to supernova remnants, the dazzling display of Wolf-Rayet ejected bubbles of emissions surrounding them was thought about, but from the sample of WR stars so far observed, these structures are only a few arcseconds across and contain a much flatter spectral index.

A potential intermediate cluster of galaxies with a large gravitational field could be another reason, as they could have been creating these giant radio versions of the Einstein rings, but the symmetry associated with ORCs and lack of cluster detections means this idea was ruled out as well.

In more contemporary terms, even binary neutron star mergers, gamma-ray bursts, and fast radio bursts have been considered because of the spherical shape these transient events can produce  – but astronomers are still not sure about these phenomena, with the proximity of ORC-2 and ORC-3 to each other potentially showcasing something new altogether that could involve these transients. 

We’re sorry to report that giant crop circles created by aliens never even made the list either.

Stumbling Across The Unknown, Unknowns

Bärbel and her team estimate that there should be roughly 1,000 ORCs across the whole sky, so for now, the search continues and more discoveries are needed. With more ORCs observed, astronomers can start to learn about where and when they are occurring, what might be causing them, and why they are structured the way we see them.

“We are looking forward to analysing our high-resolution radio continuum observations with the MeerKAT telescope as well as continuing follow-up observations with the ATCA at higher frequencies,” she said. 

“3D simulations of ORC formation scenarios are underway. And while we do all that, we keep searching for more ORCs.”

“Several of us are vigorously pursuing these,” added Ray. 

“The important next steps are (a) to find more of them, so we can tell what is important and what is a coincidence, (b) developing the theoretical models to see if they can be brought into agreement with the data, (c) observing the putative host galaxies at other wavelengths to see if we can see what is odd about them, (d) getting better radio data to see if we can measure things like magnetic fields in them,” he said.  

With machines and algorithms now learning to run through copious amounts of archival data collected by telescopes historically, there might be some opportunity to start to test cross-reference searches that look for these faint, circular structures in radio frequencies and potential elliptical galaxies in their core regions.

Due to the steepness of the spectral index that ORCs present, low-frequency telescopes like LOFAR could also potentially offer some further discoveries of these objects.

But in the end, and to this day, it’s rather an evocative thought to know that this is all still a mystery and that there are a bunch of astronomers out there using some very powerful and sophisticated equipment to search the skies whilst they scratch their heads and ask themselves, “What is it?”

And that mystery, that chase, that thrill – is the perfect science for us all. 


Video Credit: CSIRO

We acknowledge the Wajarri Yamatji as the traditional owners of the Murchison Radio-astronomy Observatory site.

The paper is available on the preprint server, arXiv.org