The study of supermassive black holes, particularly those residing at the centers of massive galaxies, has long been a pivotal topic in astrophysics. One of the most intriguing targets for such studies is Messier 87 (M87), a giant elliptical galaxy located approximately 53 million light-years away in the Virgo Cluster. At the heart of M87 lies a supermassive black hole, known as M87*, which has captivated the scientific community for its extreme characteristics and the potential it offers in understanding black hole physics, especially at scales close to their event horizons. The groundbreaking work of the Event Horizon Telescope (EHT) Collaboration has significantly advanced our understanding of M87*, especially following their 2017 and 2018 observations.
Advancements from the 2017 and 2018 EHT Observations
The 2017 EHT observations were monumental, providing the very first direct image of a black hole’s shadow, offering profound insight into the cosmic giants at the heart of galaxies. Building upon these findings, the EHT collaboration sought to deepen their understanding of M87* through follow-up observations in 2018, which further refined the initial results and enabled more detailed modeling of the black hole’s complex environment.
A key highlight of the 2018 study, published in Astronomy & Astrophysics, is the new set of simulations used to model the black hole’s accretion flow—the matter spiraling toward the black hole. These models are three times larger than those from earlier studies, incorporating a vastly more detailed view of the turbulent processes at the black hole’s accretion disk. This improvement has allowed researchers to better describe the changing dynamics observed in the luminous ring around the black hole, furthering the ability to assess how magnetic fields, accretion, and plasma dynamics affect the supermassive black hole.
Christian M. Fromm, a member of the EHT theory group and a researcher at the University of Würzburg and the MPIfR, explained the significance of this broader approach: “This study highlights the significance of incorporating larger and more diverse simulation sets in the investigation of the supermassive black hole.” According to Fromm, by coupling the multi-epoch data from 2017 and 2018 with these advanced simulations, the team was able to capture the dynamic behavior of the accretion flow around M87*. The results suggest a new, more nuanced understanding of the accretion processes near the black hole and the turbulent interactions that take place in its environment.
Behavior of the Accretion Flow Around M87*
In 2018, the EHT’s improved observations confirmed the appearance of the luminous ring that was first seen in 2017, a characteristic feature of the event horizon’s shadow. This ring measures about 43 microarcseconds across, which is roughly the angular size of a coin on the surface of the moon, as observed from Earth. This diameter closely matches theoretical predictions for a black hole that is about 6.5 billion times the mass of the Sun.
One of the most remarkable aspects of the luminous ring observed in 2018 was a 30-degree counter-clockwise shift in the brightest part of the ring compared to what was seen in 2017. This shift can be attributed to turbulence in the accretion disk, which is the swirling mass of gas and plasma that spirals into the black hole. This kind of turbulence is expected when gas and magnetic fields interact in the black hole’s accretion zone, causing bright regions to change their appearance. These observations were consistent with earlier simulations of accretion dynamics and provided critical validation for the predictive power of these models.
The team used this extensive synthetic dataset, three times larger than that from 2017, to reanalyze the accretion models. The study revealed that the gas falling toward the black hole could either align with or oppose the spin of the black hole, and that the observed variability of the bright ring is more adequately explained by gas flowing in the opposite direction of the black hole’s rotation. This observation hints that the accretion flow near the event horizon is even more dynamic than previously believed, with fluctuating interactions that are crucial for a comprehensive understanding of black hole environments.
The Role of Multi-Epoch Data and Time Variability
In the context of this study, multi-epoch data—the combination of observations taken in different years—was vital. Hung-Yi Pu, an assistant professor at National Taiwan Normal University, noted that “The black hole accretion environment is turbulent and dynamic,” and emphasized the significance of treating data from 2017 and 2018 as independent measurements. This approach allowed researchers to view the black hole’s behavior from new perspectives and brought attention to its time-varying features.
The importance of time variability became evident as scientists studied how the accumulation of data over multiple years helped refine the understanding of the evolutionary processes at the black hole’s event horizon. Since black holes are believed to have complex, time-dependent behaviors, these types of multi-year investigations offer an essential framework to analyze not just static images, but the continuous changes in their environments over time. Such studies provide a unique lens into how black holes evolve, lending insights into plasma instabilities, magnetic reconnection events, and turbulence.
Complementary Observations and Advanced Techniques
The study also benefitted from a series of complementary observations conducted by the Global Millimeter VLBI Array (GMVA). The GMVA—which contributed data gathered at 3 mm wavelengths in 2018—provided a different set of data that further enriched the understanding of the black hole’s immediate environment.
Combining the EHT’s findings at 1.3 mm wavelengths with the GMVA’s 3 mm observations proved crucial. According to Thomas P. Krichbaum, a scientist at the MPIfR and a key member of the collaboration, these different wavelengths offer unique views of the same object, enhancing the ability to probe the black hole’s environment in great detail. Together, they provided an integrated picture of the structure, composition, and variability of the material swirling around the event horizon.
The simultaneous use of two different wavelength arrays contributes to a more comprehensive and accurate depiction of M87*, capturing the diverse interactions taking place within the black hole’s accretion disk. These results underscore the transformative potential of large-scale global collaborations in pushing the boundaries of observational astrophysics.
The Importance of Continuous, Global Efforts
Looking ahead, the ongoing analysis of EHT data, including data collected in 2021 and 2022, aims to further tighten the statistical constraints around M87* and shed light on the complex dynamics of accretion. As emphasized by J. Anton Zensus, the director of the MPIfR and a founding member of the EHT, “These results are based on the continuous work of the EHT and are confirmed in the investigations with the GMVA.” The research demonstrates that collaboration and shared resources between observatories and scientific institutions around the world are not only crucial but are pivotal for making progress in such challenging areas of astrophysical research.
Zensus also emphasized the vital role of the state-of-the-art technologies that make such studies possible and highlighted that global partnerships are indispensable for pushing the field forward. Achieving these advances requires sustained effort from a vast community of astronomers, theoreticians, and experimentalists working in tandem, capitalizing on the latest tools to understand complex cosmic phenomena.
Conclusion
The 2017 and 2018 observations of the supermassive black hole M87* by the Event Horizon Telescope (EHT) represent a pivotal advancement in our understanding of black holes. By leveraging larger simulation sets, multi-epoch data, and complementary observations from the Global Millimeter VLBI Array, the EHT collaboration has significantly deepened our insights into the turbulent accretion flows around M87*. The observed shifts and variations in the black hole’s accretion disk offer new perspectives on plasma dynamics and black hole spin interactions. These findings underscore the importance of global partnerships, advanced observational techniques, and continuous research to unravel the complex behavior of supermassive black holes. With ongoing data collection and further analysis, the EHT is poised to provide increasingly refined models of black hole environments, enhancing our understanding of these enigmatic cosmic entities and their profound impact on the surrounding universe.
Reference: The persistent shadow of the supermassive black hole of M87, Astronomy & Astrophysics (2025). DOI: 10.1051/0004-6361/202451296