Massive, Mysterious Circles in Space

Art Courtesy of Alondra Moreno Santana.

Scientists believe they may have found a possible explanation for a puzzling, recently discovered cosmological phenomenon. First observed in 2019, odd radio circles (ORCs) are haloes of radio waves centered around certain galaxies. At first, these strange haloes were especially perplexing because they didn’t match the known signatures of any large-scale astronomical events. There was something more unique about this phenomenon, hence its namesake.

Professor Alison Coil of the University of California San Diego and her team believe they discovered something particularly new and exciting about these ORCs. Unlike others before them, Coil’s team looked at the optical light signatures from one of the ORC galaxies instead of merely inspecting the glowing halo of signals in radio wavelengths. This turned out to be the right call, as their observations gave way to a deeper understanding of the haloes’ mysterious origins in an article recently published in Nature.

The insight arose from their previous work. Before ORCs, Coil and her team had been studying the evolution of galaxies by assessing supermassive black holes and outflowing galactic winds, which are streams of high-speed gas particles expelled due to colossal events like supernovae—large star explosions that expel gaseous matter into the surroundings. They used observational data collected from some of the most advanced telescopes in the world to create a timeline of how certain galaxies evolved. In particular, their work was concerned with the formation of “starburst” galaxies, which experience rapid star formation over a relatively short timescale, resulting in a rapid series of supernovae.

Coil and her team stumbled upon ORCs somewhat unexpectedly. During one of their scheduled research trips to the Keck Telescope in Hawaii, they capitalized on their time with the equipment and pointed the telescope at one of the special ORC galaxies. Other research had shown evidence that these ORCs behaved like three-dimensional expanding shells, which made Coil and her team wonder whether this phenomenon could be a late-stage effect of the galactic winds they had been studying. They postulated that these radio signals could be made up of gas from galactic winds being rapidly pushed outward, originally stemming from starburst explosions. Based on their hypothesis, the team captured optical wavelength images of the galaxy, which allowed them to make some new and exciting discoveries.

The most jarring observation that arose from these optical images was the presence of an abnormally large amount of shocked gas sitting within the galaxy at the heart of each ORC. This shocked gas is essentially an abundance of singly ionized oxygen gas, which is highly unusual in these types of galaxies. “Galaxies that are like that—ones that aren’t forming stars anymore—[usually] don’t have a ton of gas in them still,” Coil said. Typically, galaxies that have had many stars form and die have lost much of their gas because these colossal events push most of it out.

Coil and her team hypothesized that this shocked gas could be the long-term result of the galactic winds decoupling, or separating, and collapsing back into the galaxy after the rapid succession of supernovae ceases. Put differently, once the supernovae “shut off,” these galactic winds may fall in on themselves, causing them to turn back into the turbulent, shocked pools of gas seen in the radio telescope images. The key is the wind that continues to propagate through space after this decoupling stage. Coil and her team believe that these outflowing galactic winds may be what’s causing the ORCs. “You need an extreme starburst and extreme winds,” Coil said. “You need to be pushing a lot of mass out for a long period of time, and then it has to shut off.” A galaxy like this is uncharted territory in the field because of the unique set of conditions that must be present. 

To confirm their findings, the team reached out to Cassandra Lochhaas, an astronomer at the Space Telescope Science Institute, who was able to create a computer simulation that confirmed the physics of the hypothesis and modeled what such an event would look like. This simulation showed that the result of decoupling is twofold. Much of the gas collapses back into the galaxy, where there are large areas of evacuated space after the burst, yet some of the gas continues to flow outwards into the space surrounding the galaxy. This, they believe, is a highly plausible explanation for the radio signals observed around the special galaxies.

Though this information is a breakthrough in our understanding of ORCs, it is only just the beginning. Coil and her team were the first to observe an ORC galaxy with optical wavelengths—but even still, their team only observed in blue optical wavelengths, meaning additional wavelengths may paint a more detailed picture of how these ORCs came to be.

The researchers currently plan to collect more data on the original ORC galaxy they were concerned with, but their long-term goals extend further. This summer, they will record data on other ORC galaxies using the aptly named Very Large Telescope (VLT) in Chile in the hopes of expanding upon their current research.

Looking back on the entire process, Coil emphasized the pivotal role of collaboration in scientific discovery. “It’s a good example of [how] one person doesn’t figure everything out,” Coil said. “You need to have collaborators—people with the data need to talk to the people with the theory.” The work she conducted alongside her team and collaborators exemplifies the essential nature of data-sharing and scientific cooperation. Such collaborative efforts not only enhance the reliability of scientific findings but also make way for new questions and scientific avenues to explore.

Coil and her team underscore the importance of collaboration in understanding ORCs. Their work creatively combines concrete observational data with theoretical modeling to better understand this puzzling cosmic phenomenon. Their discovery of charged gas in one of these special ORC galaxies paves the way for further observation and modeling to solidify their hypotheses and uncover the mysteries behind these odd galactic formations.