Mysterious ORCs Shine On The Best Shots Yet (But Still Remain Mysterious)

The ‘unusual radio circles’ were first spotted in space in 2019, making them one of the most recent mysteries the universe presents us with. But astronomers aren’t going to let it go.

In 2019, while working with the Australian SKA Pathfinder Telescope on a curious phenomenon, namely a ring of radio radiation. Astronomers had never seen anything like it, and they couldn’t quite figure it out. The latter is also apparent from the name they came up with for this mysterious phenomenon: ORC. That stands for Odd Radio Circleor unusual radio circle.

Five ORCs

We are now three years later and we are aware of five of these circles of radio radiation. It suggests that ORCs are quite rare, as they have been searched thoroughly and extensively in recent years, but without much success. However, it should be noted that the ORCs are very difficult to see with conventional radio telescopes; they don’t give off a lot of radio emissions, so very powerful telescopes are needed to see them at all.

Spherical

Of course, in recent years astronomers have also puzzled over how these mysterious radio circles form. And now they have ideas about that. For example, follow-up research has shown that some of the ORCs discovered surround distant galaxies. Moreover, they are probably not strictly speaking circles, but have a spherical shape. Such a sphere can be 1 million light-years wide and thus houses a galaxy. It is therefore suspected that those central galaxies generate the radio emission in some way.

New observations

Astronomers are determined to solve the mystery of the ORCs. And so they took a closer look at the first-discovered ORC. They used the MeerKAT radio telescope. Where ASKAP is especially good at scanning large parts of the universe, MeerKAT is particularly suitable for zooming in on specific parts of it. And so for the new study, scientists pointed the powerful radio telescope at an unusual radio circle. It results in the best recordings we have of these mysterious phenomena and also reveals something new.

On the left is the observation of ASKAP that led to the discovery of the ORCs. On the right MeerKAT’s much more detailed observation of that very first ORC. Image: EMU / ASKAP / MeerKAT.

New information

For example, the MeerKAT recordings show that arcs of radio radiation can be found in the unusual radio circles. They may have formed in the wake of other galaxies that have traveled across the ORC, the researchers say. This hypothesis is further supported by the fact that the scientists can also show that the ORC is located in a fairly densely populated area: in the vicinity of the galaxy that is at the heart of the ORC, numerous galaxies can be seen and some are located even in the ORC. If the unusual radio circle was indeed – as most researchers believe – created by certain developments in the galaxy that is at the heart of most ORCs and gradually expanded, then that ORC would be doomed to encounter the nearby galaxies and thereby influenced (and thus disturbed).

This artistic impression shows how the radio circles are born after an explosion in the central galaxy. The circles – which are 16 times larger than our Milky Way – are believed to have taken 1 billion years to reach their present size, passing other nearby galaxies as well. Image: Sam Moorfield / CSIRO.

Polarization

In addition, researchers have succeeded for the first time in measuring the polarization of the radio signal of an ORC. And that’s important, because that polarization can tell more about the presence and shape of magnetic fields. In this case, the polarization of the radio signal shows that the edge of the circle in particular has a relatively strong magnetic field.

electrons

Finally, the observations also show that the radio radiation is caused by electrons racing along the magnetic field at almost the speed of light. And that’s an interesting finding. Because there are not very many processes that can bring electrons to such a high speed. And based on the latest observations, the researchers say there are only three possible explanations for the origin of the mysterious ORCs.

One

For example, the unusual radio circle may be the remnant of a massive explosion – for example, due to the merger of two supermassive heavy holes – in the center of the galaxy. In that scenario, you would expect to find an active galactic core at the heart of the galaxy today. And that is certainly the case with the three ORCs that we know for sure that they harbor a galaxy in their hearts. And thus this is a real hypothesis.

This image combines data from the MeerKAT radio telescope with optical and near-infrared data from the Dark Energy Survey. Image: J. English (U. Manitoba) / EMU / MeerKAT / DES(CTIO).

Two

But there are two more. For example, the researchers point out that galaxies — when you study them at radio wavelengths — are often characterized by two jet streams rushing in opposite directions from the galaxy’s heart. When you look at the side of such a galaxy, those jet streams – in the form of two lobe-like structures – are often very visible. But suppose we admire these galaxies not from the side, but from above or below. In that case, those lobe-like structures could suddenly be ORC-like. It’s an interesting, but not exactly the best hypothesis. Because if ORCs were disguised jet streams, you would expect them not to be as rare as is currently believed. In addition, the researchers point out that the lobe-like structures often have an irregular shape and therefore cannot easily assume an almost perfect spherical shape, even when viewed from above. That said, this hypothesis cannot be conclusively rejected at this point either, so this explanation for the ORCs remains a contender for now.

Three

The third and final possible explanation is the starburst termination shock† This creates a shock wave due to star formation. The hypothesis is supported by the fact that many new stars indeed formed a long time ago in the galaxy at the heart of the ORC under investigation. In addition, the distance between such a shock wave and the central galaxy corresponds to the estimated size of the unusual radio circle.

It is not possible to say with certainty on the basis of the new observations which of these three explanations is the correct one. More research is therefore urgently needed. During that follow-up research, the first thing that matters is that more ORCs are discovered. In this way it can be determined which properties they have in common and that is important to be able to find out exactly how they arise. Now, discovering ORCs is easier said than done, as researchers have discovered in recent years. But there is hope. For example, there are high expectations regarding the Square Kilometer Array that should arise in South Africa (and in which the Dutch government has also invested considerably). This radio telescope is still under construction, but should be able to make the first observations in the coming years. “When completed, the SKA telescope will undoubtedly discover more ORCs and tell us more about the life cycle of galaxies,” said study researcher Ray Norris. But in the meantime, the astronomers are certainly not sitting still; there is probably still plenty to discover with ASKAP and MeerKAT.