Supermassive black holes, celestial objects with masses greater than a million times that of the Sun, remain one of the greatest enigmas of the cosmos. These dark giants, located at the center of most galaxies, have an origin that has intrigued scientists for decades. Now, a team of researchers from the University of Nevada, Las Vegas (UNLV), has found compelling evidence that the supermassive black hole at the center of our galaxy, the Milky Way, known as Sagittarius A* (Sgr A*), is likely the result of a cosmic merger in the past.
The study, published in the journal Nature Astronomy, is based on recent observations from the Event Horizon Telescope (EHT), which captured the first direct image of Sgr A* in 2022. The EHT, a product of global research collaboration, synchronizes data from eight radio observatories around the world to create a virtual Earth-sized telescope. This unique observational capability has allowed scientists to explore the most intimate details of Sgr A*, providing a new perspective on its possible origin.
UNLV astrophysicists Yihan Wang and Bing Zhang utilized EHT data to search for evidence on how Sgr A* may have formed. Supermassive black holes are believed to grow through two main pathways: the accretion of matter over time or the merger of two existing black holes. The UNLV team investigated several growth models to understand the peculiar spin and misalignment of Sgr A* relative to the angular momentum of the Milky Way. Their research demonstrated that these unusual characteristics are best explained by a major merger event involving Sgr A* and another supermassive black hole, likely from a satellite galaxy.
According to Wang, the lead author of the study and a postdoctoral fellow at UNLV’s NCfA, this discovery paves the way for understanding how supermassive black holes grow and evolve. Wang adds that the high misaligned spin of Sgr A* indicates it may have merged with another black hole, drastically altering its spin amplitude and orientation.
To test this hypothesis, the researchers used sophisticated simulations that modeled the impact of a merger, considering various scenarios that align with Sgr A*’s observed spin properties. Their results indicate that a merger with a mass ratio of 4:1 and a highly inclined orbital configuration could reproduce the spin properties observed by the EHT. This analysis suggests that the merger occurred about 9 billion years ago, coinciding with the merger of the Milky Way with the Gaia-Enceladus galaxy. Zhang, a distinguished professor of physics and astronomy at UNLV and founding director of the NCfA, explains that this event not only provides evidence for the hierarchical black hole merger theory but also sheds light on the dynamic history of our galaxy.
Sgr A*, located more than 27,000 light-years from Earth at the center of the Milky Way, represents a natural laboratory for studying these astronomical phenomena. Advanced tools like the EHT provide direct images that allow scientists to test predictive theories about the evolution and growth of black holes. The ability to study these processes with unprecedented detail is crucial for advancing our understanding of astrophysics.
The researchers emphasize that the study’s findings will have significant implications for future observations with space-based gravitational wave detectors, such as the Laser Interferometer Space Antenna (LISA), which is expected to launch in 2035. LISA is designed to detect mergers of similar supermassive black holes across the universe, providing a new window to observe these cosmic events on scales never seen before.
The prospect of using LISA to detect supermassive black hole mergers opens a new era in astrophysics. So far, ground-based gravitational wave telescopes like LIGO and Virgo have detected mergers of stellar-mass black holes and neutron stars, but not supermassive black holes. With LISA’s capability to observe at longer wavelengths, scientists expect to discover a plethora of new events that will reveal even deeper insights into the evolution of galaxies and the black holes residing at their centers.
SOURCES
University of Nevada, Las Vegas
Wang, Y., Zhang, B. Evidence of a past merger of the Galactic Centre black hole. Nat Astron (2024). doi.org/10.1038/s41550-024-02358-w
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