The origins of aptly named supermassive black holes — which might weigh in at greater than 1,000,000 times the mass of the sun and reside in the middle of most galaxies — remain considered one of the good mysteries of the cosmos.
Now, researchers from the Nevada Center for Astrophysics at UNLV (NCfA) have discovered compelling evidence suggesting that the supermassive black hole at the middle of our Milky Way galaxy, often known as Sagittarius A* (Sgr A*), is probably going the results of a past cosmic merger.
The study, published Sept. 6 within the journal Nature Astronomy, builds on recent observations from the Event Horizon Telescope (EHT), which captured the primary direct image of Sgr A* in 2022. The EHT, the results of a worldwide research collaboration, syncs data from eight existing radio observatories worldwide to create an enormous, Earth-sized virtual telescope.
UNLV astrophysicists Yihan Wang and Bing Zhang utilized the information from the EHT statement of Sgr A* to search for evidence on the way it could have formed. Supermassive black holes are thought to grow either by the accretion of matter over time, or by the merger of two existing black holes.
The UNLV team investigated various growth models to know the peculiar rapid spin and misalignment of Sgr A* relative to the Milky Way’s angular momentum. The team demonstrated that these unusual characteristics are best explained by a significant merger event involving Sgr A* and one other supermassive black hole, likely from a satellite galaxy.
“This discovery paves the way in which for our understanding of how supermassive black holes grow and evolve,” said Wang, the lead creator of the study and an NCfA postdoctoral fellow at UNLV. “The misaligned high spin of Sgr A* indicates that it could have merged with one other black hole, dramatically altering its amplitude and orientation of spin.”
Using sophisticated simulations, the researchers modeled the impact of a merger, considering various scenarios that align with the observed spin properties of Sgr A*. Their results indicate that a 4:1 mass ratio merger with a highly inclined orbital configuration could reproduce the spin properties observed by the EHT.
“This merger likely occurred around 9 billion years ago, following the Milky Way’s merger with the Gaia-Enceladus galaxy,” said Zhang, a distinguished professor of physics and astronomy at UNLV and the founding director of the NCfA. “This event not only provides evidence of the hierarchical black hole merger theory but in addition provides insights into the dynamical history of our galaxy.”
Sgr A* sits at the middle of the galaxy greater than 27,000 light years away from Earth, and complex tools just like the EHT provide direct imaging that helps scientists put predictive theories to the test.
Researchers say that the findings from the study may have significant implications for future observations with upcoming space-borne gravitational wave detectors, akin to the Laser Interferometer Space Antenna (LISA), which is planned to launch in 2035 and is anticipated to detect similar supermassive black hole mergers across the universe.
