New research indicates that Sagittarius A*, the supermassive black hole at the center of the Milky Way, likely formed through the merger of two black holes.
The study, published in Nature Astronomy in September 6, 2024 by researchers from the Nevada Center for Astrophysics (NCfA), offers insights into the processes that shape supermassive black holes and the dynamic history of our galaxy. The findings build on data from the Event Horizon Telescope (EHT), which captured the first direct image of Sagittarius A* in 2022.
Sagittarius A*’s Formation and the Role of Mergers
Supermassive black holes, like Sagittarius A*, are found at the center of most galaxies, but their formation has been a long-standing mystery in astrophysics. Two main theories suggest that these black holes either grow slowly by accumulating matter or form through the merger of smaller black holes. In the case of Sagittarius A*, recent observations from the Event Horizon Telescope revealed a rapid spin and misalignment with the Milky Way’s angular momentum, suggesting that it is likely the product of a major merger event rather than gradual growth.
"The misaligned high spin of Sgr A* indicates that it may have merged with another black hole, causing a dramatic alteration in its amplitude and orientation of spin," explained Yihan Wang, lead author of the study. The team used simulations to model different growth scenarios and found that a 4:1 mass ratio merger, likely involving a satellite galaxy, best explains Sagittarius A*’s observed properties. This merger likely occurred around 9 billion years ago, shortly after the Milky Way’s merger with the Gaia-Enceladus galaxy.
Evidence from Black Hole Dynamics
The evidence for a black hole merger goes beyond the spin properties of Sagittarius A*. The researchers also noted the misalignment of the black hole’s spin with the rest of the Milky Way, suggesting an external event had altered its orientation. This finding supports the hierarchical black hole merger theory, which posits that black holes grow through successive mergers. According to Bing Zhang, co-author of the study, "This event not only provides evidence of the hierarchical black hole merger theory but also provides insights into the dynamical history of our galaxy."
The simulation results showed that a merger with a highly inclined orbital configuration could reproduce Sagittarius A*’s current characteristics. This discovery not only helps explain the peculiarities of the Milky Way’s central black hole but also offers a clearer understanding of how galaxies and their black holes evolve through interactions with other galaxies.
The Role of the Event Horizon Telescope
The breakthrough in this study was made possible by the Event Horizon Telescope (EHT), which linked together eight radio observatories across the globe to form an Earth-sized virtual telescope. In 2022, the EHT succeeded in capturing the first image of Sagittarius A*, allowing researchers to study its properties in unprecedented detail. This image provided critical data on the spin and alignment of the black hole, which helped the team test and validate their merger hypothesis.
"Our understanding of how supermassive black holes grow and evolve will greatly benefit from this discovery," said Wang. The EHT data was crucial in confirming that the unusual spin characteristics of Sagittarius A* could not be explained by standard accretion models, making a black hole merger the most likely explanation for its current state.
Future Observations and Gravitational Wave Detection
Looking ahead, the study’s findings have significant implications for future research into black hole mergers. The upcoming Laser Interferometer Space Antenna (LISA), a space-borne gravitational wave detector set to launch in 2035, will be capable of detecting gravitational waves from similar supermassive black hole mergers across the universe. The team believes that LISA will be able to detect events like the one that formed Sagittarius A* and confirm the merger rate for supermassive black holes.
"The inferred merger rate, consistent with theoretical predictions, suggests a promising detection rate of supermassive black hole mergers for space-borne gravitational wave detectors expected to operate in the 2030s," said Zhang. As more gravitational wave detectors come online, researchers expect to gather further evidence supporting the role of mergers in the formation of supermassive black holes.
Implications for Galactic Evolution
The discovery that Sagittarius A* likely formed through a merger also has broader implications for understanding the Milky Way’s evolution. Mergers between black holes are often linked to galactic collisions, and the event that formed Sagittarius A* likely played a significant role in shaping the Milky Way’s structure and angular momentum. These mergers can have a profound impact on the distribution of mass within a galaxy and affect the orbits of stars and other celestial objects.
By studying the dynamics of supermassive black holes, researchers can gain a deeper understanding of the galactic history that shaped the Milky Way. As Zhang noted, "Not only does this event back the hierarchical black hole merger theory, but it also enlightens us on our galaxy’s dynamic history." These findings highlight the complex interactions that contribute to the formation and evolution of galaxies like our own.