New radio images of strange “radio circles” that are much larger than the Milky Way

In radio astronomy, circle-shaped objects are quite common. Since diffuse ionized gas often emits radio light, objects such as supernova remnants, planetary nebulae, and even star-forming regions can create circular arcs of diffuse gas. But in 2019, astronomers started discovering radio circles they couldn’t explain, in part because they’re so big.

Known as the Odd Radio Circles, or ORCs, they measure about a million light-years in diameter, about ten times wider than the Milky Way. Observations show that ORCs are centered around an elliptical galaxy, suggesting a galactic connection. It has been suggested that ORCs are formed by shock waves triggered by a powerful event such as a gamma ray burst or a fast radio burst, but given the size of ORCs, these events should have been triggered far in the past. . And shockwaves can usually be seen in other wavelengths such as infrared and X-rays, but ORCs are only visible in radio light.

But a new study suggests other possible origins. Using the MeerKAT radio telescope, the team captured new data from one of six confirmed ORCs, creating the highest resolution image to date. This revealed ring structures never seen before. The team also measured the polarization of the ORC’s light, which helped them narrow down possible causes.

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The first ORC image (left) and this last image (right). Credit: The EMU team, using ASKAP and MeerKAT radio continuum data

According to observations, the light corresponds to that of synchrotron radiation. It happens when charged particles are captured in a magnetic field. As the particles spiral along magnetic field lines, they emit faint radio light. This often occurs with diffuse plasma, and suggests that spherical shell gas has been pushed out of the galaxy either by a rapid period of star production or by the merger of supermassive black holes.

Another interesting possibility is that ORCs are formed directly by the galaxy’s supermassive black hole. Active galactic black holes, or active galactic nuclei (AGNs) as they are commonly known, often cause jets of matter to shoot away from the black hole at near the speed of light. Over time, these can create extensive radio lobes, where long ejected plasma shines in radio light. One idea is that ORCs are caused by radio lobes oriented along our line of sight. Rather than seeing the ejected plasma as a lobe, we would see it as glowing radio arcs.

Although this study has narrowed down the possible explanations for ORCs, there is no clear solution. But the team is undertaking further studies to look for more clues. And when future radio telescopes such as the Square Kilometer Array (SKA) are completed, we will discover even more ORCs.

Reference: Norris, Ray P., et al. “MeerKAT Discovers the Physics of an Odd Radio Circle.” arXiv preprint arXiv:2203.10669 (2022).

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