Researchers used two decades of data from radio telescopes around the world to confirm the spin of the M87 galaxy by observing the wobble of its supermassive black hole jet. This discovery marks a major advance in black hole research. The supermassive black hole at the center of the M87 galaxy, famous for the first photo of a black hole's shadow, has created another first: the jet emitted by the black hole has been shown to oscillate, directly proving that the black hole is rotating.
Schematic diagram of the tilted accretion disk model. In this diagram, the black hole's spin axis is assumed to be straight up and down. The jet direction is almost perpendicular to the plane of the disk. The misalignment between the black hole's spin axis and the disk's axis of rotation triggers a forward tilt of the disk and jets. Source: Cui Yuzhu et al. (2023), IntouchableLab@Openverse and Zhijiang Laboratory
Supermassive black holes are monsters billions of times heavier than the sun that devour everything around them, including light, and are difficult to study because no information can escape from the inside. In theory, there are very few properties that we can measure. One property that may be observed is spin, but due to the difficulties involved, no direct observations of black hole spin have been made.
Twenty years of observations uncover evidence
To look for evidence of black hole spin, an international team analyzed more than two decades of observations of the M87 galaxy. This galaxy is located 55 million light-years away in the direction of Virgo. It contains a black hole with a mass 6.5 billion times greater than the sun. The first black hole shadow image captured by the Event Horizon Telescope (EHT) in 2019 is also this black hole. As we all know, the supermassive black hole in M87 has an accretion disk and a jet. The former sends matter into the black hole, and the latter ejects matter from the vicinity of the black hole at a speed close to the speed of light.
(Above) M87 jet structure at a frequency of 43GHz averaged every two years between 2013 and 2018. The corresponding year is marked in the upper left corner. White arrows indicate the jet position angle in each subfigure. (Bottom) Observed evolution of jet direction from 2000 to 2022. The green and blue points are from observational data at 22 and 43GHz respectively. The red line is the best-fitting sinusoidal curve with a period of 11 years. Source: YuzhuCui et al. (2023)
The research team analyzed 170 time periods of data collected by the East Asia VLBI Network (EAVN), the Very Long Baseline Array (VLBA), the KVN and VERA Combined Array (KaVA), and the East Asia to Italy Near World (EATING) VLBI network. More than 20 radio telescopes around the world contributed to the research.
Research results and significance
The results show that gravitational interactions between the accretion disk and the black hole's spin cause the base of the jet to wobble, or tilt forward, much in the same way that gravitational interactions within the solar system cause the Earth to tilt forward. The team successfully linked the dynamics of the jet to the central supermassive black hole, providing direct evidence that the black hole is indeed spinning. The direction change of the jet is about 10 degrees, and the forward thrust period is 11 years, which is consistent with the supercomputer theoretical simulation conducted by the National Astronomical Observatory of Japan (NAOJ) ATERUIII.
"We are excited about this major discovery," said Cui Yuzhu, the first author of the paper. Cui Yuzhu was a graduate student at the National Astronomical Observatory of Japan and later worked as a postdoctoral researcher at the Zhejiang Laboratory. Since the misalignment between the black hole and the star disk is relatively small and the proactive period is about 11 years, accumulating high-resolution data tracking the structure of M87 for more than two decades and conducting in-depth analysis are the keys to achieving this result. "
Dr. Kazuhiro Hada from the National Astronomical Observatory of Japan explained: "After successfully imaging the black hole in this galaxy using the EHT, whether this black hole is rotating has been the focus of scientists. Now, the expectation has become certain. This monster black hole is indeed rotating."
Dr. Motoki Kino of Kogakuin University, coordinator of the East Asian VLBI Network Active Galactic Nucleus Science Working Group, said: "This is an exciting scientific milestone, finally revealed through years of joint observations by an international team of researchers from 45 institutions around the world. Our observational data are in good agreement with simple sinusoids, bringing new progress to our understanding of black holes and jet systems."