Researchers have found strong evidence that suggests the supermassive black hole at the heart of the Milky Way galaxy, referred to as Sagittarius A* (Sgr A*), may have formed from a previous cosmic merger. This study expands on recent findings from the Event Horizon Telescope (EHT), which successfully captured the first clear image of Sgr A* in 2022.
The beginnings of those called supermassive black holes—some of which exceed a million times the mass of our sun and are typically located at the centers of most galaxies—remain one of the universe’s intriguing puzzles.
Currently, a team of researchers from the Nevada Center for Astrophysics at UNLV (NCfA) has found compelling evidence indicating that Sagittarius A* (Sgr A*) is likely the outcome of a prior cosmic merger.
Published on September 6 in the journal Nature Astronomy, the study relies on recent data collected from the EHT, which includes contributions from eight radio observatories around the globe, forming a virtual Earth-sized telescope to produce images of astronomical objects.
Astrophysicists Yihan Wang and Bing Zhang at UNLV explored the EHT data concerning Sgr A* to uncover how this supermassive black hole may have formed. It is believed that supermassive black holes grow either through the gradual accumulation of matter or by merging with other black holes.
The UNLV researchers looked into different growth models to comprehend the unusual rapid spin and misalignment of Sgr A* when compared to the Milky Way’s angular momentum. Their analysis showed that these rare traits are most likely explained by a significant merger between Sgr A* and another supermassive black hole, which probably came from a smaller companion galaxy.
“This discovery enhances our understanding of how supermassive black holes develop and change over time,” explained Wang, the study’s lead author and an NCfA postdoctoral fellow at UNLV. “The misaligned high spin of Sgr A* suggests it might have merged with another black hole, which greatly changed its spin orientation and strength.”
Using advanced simulations, the researchers examined the effects of a merger, testing various scenarios that fit the observed spin characteristics of Sgr A*. Their findings indicate that a merger with a 4:1 mass ratio and a highly inclined orbit could align with the spin properties observed by the EHT.
“This merger likely took place about 9 billion years ago, shortly after the Milky Way merged with the Gaia-Enceladus galaxy,” noted Zhang, a distinguished professor of physics and astronomy at UNLV and the founding director of the NCfA. “This event supports the hierarchical merger theory of black holes and sheds light on the dynamic history of our galaxy.”
Sgr A* is located in the center of the galaxy, approximately 27,000 light-years from Earth, and advanced tools like the EHT allow scientists to test their predictive theories through direct imaging.
Researchers believe that these findings will greatly influence future studies using upcoming space-based gravitational wave detectors, such as the Laser Interferometer Space Antenna (LISA), which is set to launch in 2035 and aims to detect similar supermassive black hole mergers throughout the universe.
