Since their discovery in 2019, odd radio circles (ORCs) have baffled astronomers, appearing as enigmatic rings in radio sky surveys. Now, a team of astronomers claims to have unraveled the mystery of at least one ORC, attributing its origin to a burst of stars in a distant galaxy that resulted in a colossal explosion. This explosion propelled gases into surrounding space, generating a shock wave that manifested as an ORC more than 10 times wider than the Milky Way.
Presenting their findings at the American Astronomical Society meeting in New Orleans, the astronomers revealed that the Australian Square Kilometre Array Pathfinder (ASKAP) first identified the ORCs in 2019. These peculiar phenomena, less than a dozen of which have been found to date, have defied easy explanation. Initial speculations ranged from debris expanding from a supernova explosion to radio emissions caused by central supermassive black holes in galaxies.
The breakthrough came when Alison Coil of the University of California, San Diego, and her colleagues observed an ORC known as ORC4 using one of the Keck telescopes in Hawaii. They discovered an unexpected abundance of fluorescent light from oxygen atoms (OII), indicating an unusually intense period of star formation in the galaxy at ORC4's center. This intense starburst, occurring a billion years earlier, led to the formation of massive stars that subsequently exploded as supernovae, generating a powerful galactic wind.
The galactic wind, upon colliding with the outer gas surrounding the galaxy, produced a shock wave visible as an ORC. Over time, this shock wave expanded and slowed down, allowing radio arrays to capture the synchrotron light generated by fast-moving electrons within the shock wave. The team's computer model of this process, detailed in a Nature paper, aligns with the observed data and offers a plausible explanation for the peculiar radio circles.
Moreover, the study hints at the possibility of a second shock wave occurring as the galactic wind stalls and falls back toward the galaxy, potentially explaining the intense OII emission. While the model doesn't clarify the trigger for the starburst, the researchers speculate that a merger of similar-size galaxies might have initiated the burst of star formation.
Astronomers see this discovery not only as a resolution to the ORC mystery but also as an opportunity to gain new insights into starbursts and their impact on galactic winds. ORCs, acting as relics of past outflows, offer a unique opportunity to trace gas far beyond the galaxy, providing a valuable tool for understanding the dynamics of celestial phenomena.
More: https://www.science.org/content/article/astronomers-solve-mystery-odd-radio-circle-deep-space
