Radio Telescopes Narrowing In On The Odd Radio Circles Mystery

As far as space mysteries go, this one’s a doozy. So much so that when scientists first discovered this new phenomenon, the very literal words they used to describe them were “W.T.F.?”. Since then, these large, odd, circular objects that resemble the remnants of exploded stars, or planetary nebulae, and have been observed in radio wavelengths have come to be known as, well, ‘Odd Radio Circles’ or ORCs, keeping in line with the creative nature of naming conventions by astronomers.

Now, radio astronomers have used some of the most sensitive and powerful instruments on Earth to narrow down what these mysterious objects can be. In a new paper, published today by the journal Monthly Notices of the Royal Astronomical Society, astronomers have taken another detailed image of an ORC using South Africa’s powerful MeerKAT radio telescope, helping provide some further context and narrowing down what these things could be. 

Australian astronomers first stumbled upon ORCs in 2019, when they were using the ASKAP radio telescope - a 36-dish antenna array located in Western Australia, which is owned and operated by Australia’s national science agency, CSIRO. These extragalactic sources of radio light caught the eyes of scientists as part of ASKAP’s Evolutionary Map of the Universe (EMU) survey, as they are very big and resemble giant circles of radio light surrounding distant galaxies. 

To date, only a handful have been found at radio wavelengths, with no counterpart signal from other parts of the electromagnetic spectrum (such as optical, infrared or x-ray light) yet detected. Now, with this new MeerKAT image, astronomers have narrowed down their theories to three possible outcomes.

The first is that an ORC is the remnant result of a huge explosion that occurs at the centre of host galaxies, potentially caused by the merger of two supermassive black holes (which occupy the hearts of most galaxies). We know that galaxies merge and collide over time, so are ORCs now a remnant artifact of these violent events? 

The second theory states that we could be looking at giant powerful jets of energetic particles that are blasted out from the centre of these galaxies - we have seen evidence of these types of relativistic jets from other supermassive black holes in active galaxies such as the beautiful southern target Centaurus A, or the extremely massive M87 (infamously known for the first image of a supermassive black hole ever captured). 

And for the last theory, astronomers believe it might be the result of an expanding bubble of gas, the edge of a termination shock, that is resultant from a big burst of star formation and production within the host galaxies. 

“People often want to explain their observations and show that it aligns with our best knowledge. To me, it’s much more exciting to discover something new, that defies our current understanding,” says Dr Jordan Collier of the Inter-University Institute for Data-Intensive Astronomy, who compiled the image from MeerKAT data.

As only five ORCs have been found to date, they still remain rather elusive to the radio astronomy community. We know that they only appear in radio light and that the central galaxies are at large distances (out to redshifts of 0.3 - 0.5, roughly 4.1 - 6.8 billion light-years away). 

They have diameters that are extremely large, about one million light-years across which when put another way, translates to about 16 times the diameter of our Milky Way Galaxy. To give some context, the distance between the Sun and Neptune is a mere four light-hours, which from our Earthly perspective, seems extraordinarily distant. 

Since their discovery, scientists have considered ORCs to be a variety of objects, such as giant galactic radio lobes, huge blasts waves generated by merging supermassive black hole binaries, and/or interacting galaxies. They’ve also been able to rule out reasons such as imaging artifacts, lens flaring or glitches. 

These new findings and data from MeerKAT of the object known as ORC-1 will now help further refine what astronomers think might be creating these objects, closing in on their role in the Universe and in particular, any link to galactic evolution and structure. 

“We know ORCs are rings of faint radio emissions surrounding a galaxy with a highly active black hole at its centre, but we don’t yet know what causes them, or why they are so rare,” added Professor Ray Norris from Western Sydney University and CSIRO, one of the authors on the paper.

ORC-1 was originally observed with ASKAP at 944 MHz but has since also been observed with a number of other Australian telescopes and at different frequencies, including the CSIRO ATCA at 2.1 GHz, and the Curtin University / ICRAR MWA telescope (also located near ASKAP) at 88 - 154 MHz. 

As part of this latest study using MeerKAT, the object was imaged in high resolution revealing much more internal detail of the structure than had been previously observed. Additionally, data about its polarisation and spectral index, as well as the properties of the central host galaxy have now been further analysed, helping to narrow down to the three hypotheses about what these objects could be. 

Whilst the discovery of ORCs was made by ASKAP, this latest research and image, captured by MeerKAT, reflects how both telescopes are working together towards the ambitious and overarching science goals of the SKA telescope. 

Once completed, this behemoth will reside across two continents (Australia and South Africa) and be one of the biggest, most complicated and most powerful scientific instruments that humans have ever built. 

The SKA will feature two main types of radio telescopes including over 131,000 of the Christmas-tree-looking antennas located here in Australia, which will scan the sky in the low-frequency range of 50 MHz - 350 MHz, and 197 of the dish antennas located over in Africa, which will have a frequency range of 350 MHz - 15 GHz. 

Working collaboratively, the SKA plans to try and find answers to some of astronomy’s biggest questions, such as why the Universe’s expansion is accelerating, the testing of Einstein’s theory of gravity, and providing further context to the question that we all consider at one point in our lives - are we all alone in the Universe?

“Nearly all astronomy projects are made better by international collaboration – both with the teams of people involved and the technology available,” said Professor Elaine Sadler, Chief Scientist of CSIRO’s Australia Telescope National Facility, which includes ASKAP.

“ASKAP and MeerKAT are both precursors to the international SKA project. Our developing understanding of odd radio circles is enabled by these complementary telescopes working together,” said Professor Sadler.

“No doubt the SKA telescopes, once built, will find many more ORCs and be able to tell us more about the lifecycle of galaxies,” said Professor Norris.

 “Until the SKA becomes operational, ASKAP and MeerKAT are set to revolutionise our understanding of the Universe faster than ever before,” he concluded.

We acknowledge the Wajarri Yamatji as the traditional owners of the Murchison Radio-astronomy Observatory site in Western Australia, whilst MeerKAT is located on the traditional lands of the San and Khoi people in the Northern Cape Province of South Africa.