Scientists have spotted yet another bizarre, gigantic, and unexplained circle-shaped radio structure in outer space, a discovery that contributes to “exciting times in astronomy,” reports a new study.
The bubble is the latest example of an Odd Radio Circle (ORC), an aptly named type of spectral ring that debuted in a 2020 paper led by Western Sydney University astrophysicist Ray Norris. Norris and his colleagues detected four of these enormous circles eerily glowing in faint radio wavelengths far beyond our galaxy.
Now, scientists led by Bärbel Koribalski, a research scientist at CSIRO's Australia Telescope National Facility, have discovered a fifth ORC that appears to span about one million light years.
This structure, named ORC J0102–2450, also looks like it has an elliptical galaxy at its center, a feature it shares with two of the ORCs found by Norris’ team. Koribalski and her co-authors, including Norris, said the presence of the galaxies is “unlikely a coincidence” and may help explain the origin of these ghostly rings, according to their forthcoming study in the Monthly Notices of the Royal Astronomical Society Letters, which is available on the preprint server arXiv.
ORCs have flown under the radar for decades because they are extremely dim, but new and advanced radio telescopes, such as the Australian Square Kilometre Array Pathfinder (ASKAP), are sensitive enough to spot the huge bubbles.
“Since ASKAP can see an area of 30 square degrees—a very large area—it is an excellent survey instrument, and the data collected will lead to many more discoveries,” Koribalski said in an email. “We typically observe one 30 square degree region for ~10 hours (if possible), then repeat these observations multiple times to increase the sensitivity of the field. The more sensitive, the more radio sources we detect.”
Koribalski and her colleagues combined eight ASKAP observations conducted between Aug 2019 and Dec 2020 to catch a glimpse of the newest member of the ORC family.
Because ORCs are so faint, it’s difficult to judge exactly how far they are from Earth, which complicates estimates of their size. However, the pattern of galaxies showing up smack-dab in the center of ORCs, now observed in three out of five cases, could provide a distance measurement. For instance, the galaxy in the middle of the newly discovered ORC is about three billion light years away, suggesting that the circle encompasses an expanse of about 980,000 light years.
This distant galaxy may also be the source of the giant bubble, according to the study. Some galaxies are flanked by long radio-bright lobes forged by jets from their galactic cores, which spew energetic gas tens of thousands of light years into intergalactic space. Koribalski and her colleagues speculate that ORC J0102–2450 may be one of these lobes, viewed end-on from Earth, as it rapidly fades after the jets have turned off.
The team also suggests that “a giant blast wave in the central galaxy” that produces “a spherical shell of radio emission” might be the progenitor of ORCs, according to the study. One possible trigger of such a large-scale spherical shock could be the merger of two supermassive black holes during a galactic collision.
Astronomers have seen countless examples of galaxies crashing into each other—indeed, our own Milky Way is expected to collide with Andromeda in a few billion years—making this a plausible explanation for the ORCS.
The researchers proposed a third outlier scenario in which the central galaxy is interacting with another galaxy inside the ring, located several thousand light years to the southeast of the center. However, the team noted in the study that it “seems unlikely that two interacting radio galaxies would be able to form the observed radio ring.”
Ultimately, the mystery of the ORCs can only be resolved with more observations and simulations that can constrain the explanations presented by Koribalski and her colleagues.
“It would be cool if we have detected a totally new phenomenon,” Koribalski said. “ORCs are rare. We should see smaller and bigger ones, but currently only see one size. So, I hope my paper will encourage other groups around the world to search for ORCs in their data.”