A Bubbly Origin for Odd Radio Circles

Stumped by Space ORCs

ORCs are faint extragalactic circles of radio emission that appear to be invisible at other wavelengths. As the number of known ORCs slowly climbs, researchers have begun to test possible formation mechanisms. Among the many possibilities are the jets of active galactic nuclei: luminous galactic centers powered by accreting supermassive black holes.

In this hypothesis, active galactic nucleus jets filled with fast-moving charged particles carve out bubbles in the surrounding gas of the intracluster medium. When the cosmic rays smash into the intracluster gas, they produce electrons and positrons, which spiral around tangled magnetic field lines and emit radio waves. If the jets are viewed precisely on-axis, the resulting radio bubbles would be circular. Previous simulations of active galactic nucleus jets colliding with surrounding gas have created bubbles, but these bubbles haven’t reached the size of ORCs, which are hundreds of thousands or millions of light-years across.

Now, Yen-Hsing Lin (National Tsing Hua University) and Hsiang-Yi Karen Yang (National Tsing Hua University and National Center for Theoretical Sciences, Taiwan) have shown how active galactic nuclei can blow ORC-sized bubbles.

Galaxies Blowing Bubbles

Using three-dimensional magnetohydrodynamics simulations, Lin and Yang shot bipolar jets into intracluster gas, tracking the jet evolution and bubble formation across 200 million years of simulation time. The team focused on two of their simulation runs, which created bright-edged circles of radio emission that are roughly the size of known ORCs.

These simulated ORCs arise from jet-inflated bubbles viewed straight down the axis, as predicted, but further simulations showed that the bubbles produce ORCs when viewed up to 30 degrees off-axis. (At larger angles, they still produce intriguing radio structures but lose their distinct circular shape.) This result relaxes the requirement that the jets be viewed exactly on-axis.

A critical factor in determining whether a galaxy’s jets produce an ORC is the ability of the tenuous jets to completely excavate the higher-density intracluster gas that it interacts with. For this reason, low-mass galaxy clusters, which contain less intracluster gas, may be more likely to host ORCs.

Another Way of Looking

Lin and Yang explored other characteristics that could support the jet-blown bubble hypothesis. They found that the simulated radio rings are clumpy, varying in brightness around the ring, which could potentially be seen in high-resolution observations.

As for observations outside radio wavelengths, the team found that only certain ORCs would produce enough X-rays to be picked up by the Chandra X-ray Observatory, and even those would require 11.5 days of observing time. Based on these results, it’s unsurprising that ORCs have so far appeared invisible at X-ray wavelengths — but that might change. Two X-ray telescopes slated to launch in the 2030s, NASA’s Advanced X-ray Imaging Satellite (AXIS) and the European Space Agency’s Advanced Telescope for High-ENergy Astrophysics (Athena), could reduce the necessary observing time to just 4 hours.

Going forward, Lin and Yang aim to continue their simulations, investigating the absolute brightness, polarization, and other properties of the radio emission, allowing for better comparisons with observations and a greater understanding of ORC origins.

Citation

“Active Galactic Nucleus Jet-Inflated Bubbles as Possible Origin of Odd Radio Circles,” Yen-Hsing Lin and H.-Y. Karen Yang 2024 ApJ 974 269. doi:10.3847/1538-4357/ad70af

For more on ORCs, read Monica Young’s feature article in the January 2025 issue of S&T.