Researchers from the Nevada Center for Astrophysics at UNLV (NCfA) have uncovered new evidence that the supermassive black hole at the center of the Milky Way, called Sagittarius A' (Sgr A'), may have formed as a result of a cosmic merger. The study was published on September 6 in 'Nature Astronomy'.

The research builds on data from the Event Horizon Telescope (EHT), which took the first-ever image of Sgr A' in 2022. The EHT combines data from eight radio observatories across the globe to simulate a massive telescope the size of Earth.

UNLV astrophysicists Yihan Wang and Bing Zhang used this data to study Sgr A''s formation. Supermassive black holes can grow by accumulating matter over time or by merging with other black holes. The team analyzed models to explain Sgr A''s unusual rapid spin and its misalignment with the Milky Way's angular momentum.

Their study suggests that these features are best explained by a major merger between Sgr A' and another supermassive black hole, likely originating from a satellite galaxy. "This discovery paves the way for our understanding of how supermassive black holes grow and evolve," said Wang, the lead author and NCfA postdoctoral fellow. "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."

The researchers conducted simulations to model the effects of such a merger, finding that a 4:1 mass ratio merger with a highly inclined orbit could account for Sgr A''s current spin properties.

"This merger likely occurred around 9 billion years ago, following the Milky Way's merger with the Gaia-Enceladus galaxy," explained Zhang, a distinguished UNLV professor and NCfA director. "This event not only provides evidence of the hierarchical black hole merger theory but also provides insights into the dynamical history of our galaxy."

Sgr A', located more than 27,000 light years away, continues to be studied with advanced tools like the EHT. These observations help confirm theories about the formation and evolution of supermassive black holes.

The study's findings could impact future research, particularly with upcoming missions like the Laser Interferometer Space Antenna (LISA), a gravitational wave detector set to launch in 2035, which could detect similar mergers throughout the universe.