Discovery of oscillating jet in radio galaxy M87 confirms black hole spin

M87, a nearby radio galaxy situated 55 million light-years away from Earth, houses a black hole that’s a staggering 6.5 billion times more massive than our Sun. An international team, led by Chinese researcher Dr. Cui Yuzhu, conducted a groundbreaking study, published in Nature on September 27, using a global network of radio telescopes.

By meticulously analyzing telescope data spanning from 2000 to 2022, the research group unveiled a recurring 11-year cycle in the motion of the jet at the heart of M87. This phenomenon aligns with Einstein’s General Theory of Relativity and offers conclusive proof of the black hole’s spin.

At the center of active galaxies, supermassive black holes wield incredible gravitational force, drawing in vast amounts of matter and expelling powerful plasma jets that can stretch thousands of light-years across the cosmos.

The dynamics of energy transfer between these black holes, their accretion disks, and the relativistic jets have perplexed scientists for a century. A prevalent theory posits that a spinning black hole can extract energy, propelling nearby material with immense force. However, until now, the spin of supermassive black holes, a pivotal aspect of this process and second only to mass in importance, remained unobserved.

The study centered on M87, where the first astrophysical jet was observed in 1918. Thanks to its proximity, the regions where these jets form near the black hole can be scrutinized in detail using Very Long Baseline Interferometry (VLBI), exemplified by the Event Horizon Telescope’s recent black hole shadow imaging. Over 23 years of VLBI data from M87, the team detected the periodic precession of the jet at its base, shedding light on the black hole’s status.

At the core of this discovery lies a profound question: What cosmic force can alter the trajectory of such a potent jet? The answer may be concealed within the behavior of the accretion disk, a structure intimately linked to the central supermassive black hole.

As matter spirals toward the black hole due to its angular momentum, it forms a disk-like structure before ultimately succumbing to the black hole’s gravitational pull. However, if the black hole is spinning, it warps the surrounding spacetime significantly, causing nearby objects to follow its rotational axis—an effect known as “frame-dragging,” a concept predicted by Einstein’s theory of relativity.

Top panel: M87 jet structure at 43 GHz based on bi-yearly stacking data observed from 2013–2018. The white arrows indicate the jet position angle in each subplot. Bottom panel: Best fitted results based on the yearly stacked image from 2000–2022. The green and blue points were obtained from observations at 22 GHz and 43 GHz, respectively. The red line represents the best fit according to the precession model. Credit: Yuzhu Cui et al., 2023

The research team’s thorough investigation has revealed a fascinating alignment phenomenon within the M87 galaxy. They found that the rotational axis of the accretion disk, the swirling matter around the black hole, doesn’t quite match the spin axis of the supermassive black hole itself. This misalignment leads to the formation of a precessional jet, a crucial discovery that unequivocally proves the black hole’s spin. This insight significantly enhances our comprehension of supermassive black holes.

Dr. Cui Yuzhu, a postdoctoral researcher at Zhejiang Lab in Hangzhou and the lead author of the paper, expressed their excitement, emphasizing that this discovery was made possible through two decades of accumulating high-resolution data and meticulous analysis. The subtle misalignment and the 11-year precession period demanded extensive efforts.

Dr. Kazuhiro Hada from the National Astronomical Observatory of Japan, reflecting on the long-standing question of whether this black hole was spinning, noted that the anticipation has now transformed into certainty. This colossal black hole, it turns out, is indeed in rotation.

This groundbreaking research relied on 170 epochs of observations from various observatories, including the East Asian VLBI Network (EAVN), the Very Long Baseline Array (VLBA), the joint array of KVN and VERA (KaVA), and the East Asia to Italy Nearly Global (EATING) network. Over 20 telescopes worldwide contributed to this study.

China’s radio telescopes played a vital role in this project. Notably, the Tianma 65-meter radio telescope, with its impressive dish and sensitivity at millimeter wavelengths, made significant contributions. The Xinjiang 26-meter radio telescope enhanced the angular resolution of EAVN observations. The future addition of the Shigatse 40-meter radio telescope, based at the Shanghai Astronomical Observatory, promises further advancements in millimeter imaging capabilities, particularly in the sub-millimeter range.

Professor Shen Zhiqiang, Director of the Shanghai Astronomical Observatory of the Chinese Academy of Sciences, highlighted the ideal conditions for (sub-)millimeter wavelength observations in the Tibetan Plateau, where the Shigatse 40-meter radio telescope is located. This aligns with their goal to bolster domestic sub-millimeter facilities for astronomical research.

While this study uncovers the enigmatic properties of supermassive black holes, it also presents formidable challenges. The structure of the accretion disk and the precise measurement of the M87 supermassive black hole’s spin remain areas of uncertainty. Furthermore, this work suggests that similar configurations may exist in other celestial sources, presenting an exciting challenge for scientists to detect and understand them.

Source: Chinese Academy of Sciences

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