Reviewed by Danielle Ellis, B.Sc.Sep 9 2024
Researchers from the Nevada Center for Astrophysics at UNLV (NCfA) have found strong evidence in a study published in Nature Astronomy that suggests Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way galaxy, is most likely the product of a previous cosmic merger.
The first image of Sagittarius A*, the supermassive black hole at the center of our galaxy. It was captured by the Event Horizon Telescope (EHT), an array which linked together eight existing radio observatories across the planet to form a single “Earth-sized” virtual telescope. Image Credit: EHT Collaboration
One of the greatest mysteries of the universe is the formation of the appropriately termed supermassive black holes, which can weigh up to a million times the mass of the sun and are found in the center of most galaxies.
The analysis is based on recent observations from the Event Horizon Telescope (EHT), which obtained the first direct image of Sgr A* in 2022. The EHT, the outcome of a multinational scientific effort, combines data from eight existing radio observatories to form a huge, Earth-sized virtual telescope.
UNLV astrophysicists Yihan Wang and Bing Zhang used data from the EHT observation of Sgr A* to investigate how it developed. Supermassive black holes are hypothesized to form by the accretion of matter over time or the merger of two existing black holes.
The UNLV team studied alternative development theories to better comprehend Sgr A*’s unusual quick spin and misalignment with the Milky Way’s angular momentum. The scientists proved that these peculiar traits are best described by a huge merger event between Sgr A* and another supermassive black hole, most likely from a satellite galaxy.
This discovery paves the way for our understanding of how supermassive black holes grow and evolve. The misaligned high spin of Sgr A* indicates that it may have merged with another black hole, dramatically altering its amplitude and orientation of spin.
Yihan Wang, Study Lead Author and NCfA Postdoctoral Fellow, University of Nevada, Las Vegas
The researchers used extensive simulations to simulate the consequences of a merger, taking into account alternative scenarios that corresponded to the reported spin characteristics of Sgr A*. Their findings suggest that a 4:1 mass ratio merger with a highly inclined orbital configuration might replicate the spin characteristics found by the EHT.
This merger likely occurred around 9 billion years ago, following the Milky Way’s merger with the Gaia-Enceladus galaxy. This event not only provides evidence of the hierarchical black hole merger theory but also provides insights into the dynamic history of our galaxy.
Bing Zhang, Distinguished Professor, Physics and Astronomy, University of Nevada, Las Vegas
Sgr A* is located in the core of the galaxy, more than 27,000 light-years from Earth. Modern technologies such as the EHT allow direct imaging, allowing scientists to test predicted ideas.
According to the researchers, the study’s findings will significantly affect future observations with forthcoming space-borne gravitational wave detectors, such as the Laser Interferometer Space Antenna (LISA), which is set to launch in 2035 and is anticipated to identify similar supermassive black hole mergers across the universe.
Journal Reference:
Wang, Y. and Zhang, B. (2024) Evidence of a past merger of the Galactic Centre black hole. Nature Astronomy. doi.org/10.1038/s41550-024-02358-w