New research suggests 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 found that the misaligned high spin of Sagittarius A* indicates a major merger event rather than gradual growth.
The researchers 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.
The evidence for a black hole merger goes beyond the spin properties of Sagittarius A*. The misalignment of the black hole’s spin with the rest of the Milky Way suggests 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.
The Event Horizon Telescope (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 research into black hole mergers will greatly benefit from this discovery. The upcoming Laser Interferometer Space Antenna (LISA) will be capable of detecting gravitational waves from similar supermassive black hole mergers across the universe.
The study’s findings have significant 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.
Source: https://dailygalaxy.com/2024/09/supermassive-black-hole-through-merger/