A team of international researchers involved with the Event Horizon Telescope (EHT), which includes South Korean scientists, revealed the reason for the distorted shape of the ring around the 'M87' black hole. It was not due to the black hole's gravity or rotation, but rather the turbulence of matter swirling around the black hole.
The EHT research team published a research paper on the 10th in the international journal "Astronomy & Astrophysics" detailing the reasons behind the asymmetry of the M87 black hole ring. The study involved 12 domestic researchers, including the lead author, Jo Il-je, a postdoctoral researcher at Korea Astronomy and Space Science Institute (KASI) and Yonsei University.
The supermassive black hole located at the center of the M87 galaxy represents the first-ever image of a black hole captured in human history. Since the first release of the M87 black hole image in 2019, astronomers around the world have been dedicated to studying why the shadow ring of the black hole appears somewhat elongated.
Einstein's theory of gravity predicted that the shadow of a black hole would be somewhat distorted into an elliptical shape due to the curvature of spacetime caused by the black hole's rotation. Therefore, measuring this ellipticity serves as direct evidence of the black hole's rotation and is also a major topic for verifying the general theory of relativity.
The research team obtained results from observations in 2018 that included the Greenland telescope added to the existing EHT observations. The measurement precision improved compared to previous observations, allowing for an ellipse of the black hole ring to be measured 3 to 5 times more accurately than before. The observation results showed a ring that deviated by about 8% from a perfect circle. This ellipse was tilted 50 degrees counterclockwise from the north, which well aligned with the direction of the brightest part of the ring.
To verify whether this elliptical shape was due to the black hole's rotation, the research team compared their findings with various theoretical simulations. As a result, they confirmed that there is no statistically significant correlation between the black hole's rotation and the observed ellipticity.
Instead, the ellipticity correlated with models possessing jets, which are fast flows of matter emitted from the black hole. This indicates that the shape of the ring cannot be explained solely by gravity or rotation, but is determined by the movements of matter around the black hole.
Dr. Jo Il-je, a co-first author of this research paper, noted, "I am proud to have revealed that the reason for the distortion of the black hole ring is more greatly influenced by the turbulent plasma circulating around the black hole than by the black hole's rotation, contrary to previous predictions." He also expressed hope that with the spread of the Korea Very Long Baseline Interferometry (KVN) multi-frequency simultaneous observation reception system being introduced at the EHT, it will be possible to obtain even more precise images of black holes.
References
Astronomy & Astrophysics (2025), DOI: https://www.aanda.org/10.1051/0004-6361/202555235