Unexplained odd radio circles in the sky are detected by the ASKAP and MeerKAT radio telescopes.
The Square Kilometre Array Telescope has been imagined, planned and designed by radio astronomers for over three decades. This effort has led to new technologies and a generation of scientific instruments that are able to detect phenomena in the universe that have never been seen before. Today, collaboration between the two SKA host countries of Australia and South Africa are enabling astronomers to better understand one of the newest, never-before seen astronomical objects, odd radio circles.
What are odd radio circles?
These near-perfect circular structures glow in radio waves but are very faint, which is why they have gone undetected until now. It is also clear that they cannot possibly be part of our own Milky Way galaxy. They are much, much further away. Of the five odd radio circles discovered so far, three are similar and can be reliably associated with known galaxies located at the centres of the odd radio circles. The host galaxies at the centres of those odd radio circles are between 3 and 5.5 billion light years away. Astronomers infer from this that the radio light forming the odd radio circles are probably emitted from expanding spherical shells around the host galaxies. If so, those shells would be 750 million to 1.3 billion light years across, much larger than the largest known galaxies in the universe.
What telescopes are involved?
The first odd radio circles were discovered using the Australian Square Kilometre Array Pathfinder (ASKAP) telescope in Australia. To confirm the discoveries, other radio telescopes have looked at the same directions in the sky and seen the circles. One odd radio circle has subsequently been discovered using the Giant Metrewave Radio Telescope (GMRT) in India. In total, five odd radio circles have been found.
What has this study done?
In this project, astronomers used the South African MeerKAT telescope to observe the first odd radio circle ever discovered, in great detail. The ability of the telescope to detect very faint signals at high resolution has opened a new window into odd radio circles. For the first time ever, astronomers have been able to measure the polarisation of the radio signal of the odd radio circle. Polarisation is an indication of the presence and shape of magnetic fields. They have also measured something called a spectral index for the first time in an odd radio circle. The spectral index links the strength of the radio emission with radio frequency and is a strong indicator of what underlying phenomenon is the source of the observed radio light.
Thanks to these world-first observations, the scientists involved have made impressive progress in understanding what phenomenon could be at the origin of these vast circular structures called odd radio circles.
What does this odd radio circle look like?
This figure shows the impressive improvement in imaging the odd radio circle. In particular, this is the first time that structures within the circle are observed.
In this beautiful image, the odd radio circle and associated radio light is visible in mint green and is superimposed on an image in visible light of the same area of the sky from previous observations. This shows that what is seen in radio is often different from what is seen in optical light – the light that our eyes can see.
What have they found?
This odd radio circle is found in an area that has a high density of galaxies, meaning that there are many other galaxies in the neighbourhood. Many of those nearby galaxies are known from other observations. Their distances have been estimated. It places the surrounding galaxies in the vicinity of the host galaxy of the odd radio circle. Some are even inside the odd radio circle. If the odd radio circle is due to an expanding spherical shell, the presence of those galaxies as obstacles on the path of the expanding shell explains the slight irregularities seen in the new image of the odd radio circle compared to an unperturbed circle.
Astronomers have also found that the polarisation of the light in the odd radio circle follows the contours of the odd radio circle. This matches what would be expected from a magnetic field in a cluster of galaxies as is seen in the neighbourhood of the odd radio circle.
The spectral index data tells the astronomers the radio light is due to electrons travelling at nearly the speed of light in the ambient magnetic field present amidst the galaxies.
This is where the understanding of the underlying mechanism propelling this shell of energy in such a near-perfect shape over intergalactic distances comes into play. Those electrons could have been accelerated by a few processes. Astronomers are trying to use observations of the odd radio circle and its host galaxy to tell those processes apart.
Their reasoning leaves three possible origins for the expanding shell and the faint radio signal forming the odd radio circle. All three are analysed in the paper and it is not yet possible to distinguish clearly which is the cause of the shell, so the three potential explanations are:
1- The odd radio circle is the remnant of a massive explosion at the centre of the host galaxy, like the merger of two supermassive black holes. If this is the case, the result of the merger would be seen in radio light in the form of an active galactic nucleus, or AGN. And in fact, the radio emission of all three odd radio circles with a known host galaxy at the centre, indicate that the host galaxies do indeed have active galactic nuclei, AGNs.
2- Galaxies observed in radio light most often show two powerful jets emerging from the centre of the galaxy. Those two jets are very clear when observed sideways, forming two opposing lobes of radio light, but what if odd radio circles were the same lobe structures, just seen head-on? This could work but that would lead to the centre of the host galaxy being much brighter unless it’s gone dim since. Moreover, there should be many more such end-on radio lobe observations among all radio galaxies known but there are only five observed odd radio circles. Finally lobes tend to have irregular shapes, making the impressive circular shape unlikely. Nonetheless this explanation cannot not be completely ruled out with the current data.
3- A starburst termination shock. This is a form of wind of high speed, high energy particles like electrons, that forms in a galaxy in which a lot of star formation occurs. The observations of the odd radio circle, made by ASKAP and MeerKAT, as well as other data sources, seem to indicate that the host galaxy did indeed form a lot of new stars a few billions of years ago, but not anymore. This origin hypothesis of the odd radio circle is reinforced by the fact that the distance of the shockwave of such a wind from the host galaxy at its centre corresponds with the estimated size of the odd radio circle around its host galaxy.
So which is it? While it is not yet possible to determine exactly what causes these odd radio circles to form, these new observations have shed incomparable light on a phenomenon that astronomers worldwide have just newly discovered.
With the Square Kilometre Array under construction, even more odd radio circles are expected to be discovered, and even more detailed observations will come along. This new generation of radio telescopes doesn’t just let us see the universe in completely unprecedented detail, but the SKA Observatory will need talented scientists to make those discoveries. This is but the beginning of a revolutionary journey giving us new understandings of our universe and our place within it.