Study finds evidence of origin of supermassive black hole at center of galaxy

The origins of supermassive black holes, which can weigh more than a million times the mass of the Sun and reside at the centers of most galaxies, remain one of the great mysteries of the cosmos.

Today, researchers at UNLV’s Nevada Center for Astrophysics (NCfA) have discovered compelling evidence suggesting that the supermassive black hole at the center of our Milky Way galaxy, known as Sagittarius A* (Sgr A*), is likely the result of a past cosmic merger.

The study, published September 6 in the journal Astronomy of naturebuilds on recent observations from the Event Horizon Telescope (EHT), which captured the first direct image of Sgr A* in 2022. The EHT, a global research collaboration, synchronizes data from eight existing radio observatories around the world to create a massive, Earth-sized virtual telescope.

UNLV astrophysicists Yihan Wang and Bing Zhang used data from the EHT observation of Sgr A* to look for evidence of how it might have formed. Supermassive black holes are thought to grow either by accreting matter over time or by merging two existing black holes.

The UNLV team studied various growth models to understand Sgr A*’s unusual rapid rotation and misalignment with the Milky Way’s angular momentum. The team demonstrated that these unusual features are best explained by a major merger involving Sgr A* and another supermassive black hole, likely from a satellite galaxy.

“This discovery paves the way for a better understanding of the growth and evolution of supermassive black holes,” said Wang, lead author of the study and NCfA postdoctoral researcher at UNLV. “The high, misaligned spin of Sgr A* indicates that it may have merged with another black hole, significantly changing its amplitude and spin orientation.”

Using sophisticated simulations, the researchers modeled the impact of a merger, considering various scenarios that match the observed spin properties of Sgr A*. Their results indicate that a merger with a mass ratio of 4:1 and a highly inclined orbital configuration could reproduce the spin properties observed by the EHT.

“This merger likely occurred about 9 billion years ago, after the Milky Way merged with the Gaia-Enceladus galaxy,” said Zhang, a UNLV Distinguished Professor of Physics and Astronomy and founding director of the NCfA. “This event not only provides evidence for the theory of hierarchical black hole mergers, but also provides insight into the dynamic history of our galaxy.”

Sgr A* lies at the center of the galaxy, more than 27,000 light-years from Earth, and sophisticated tools like the EHT provide direct imaging that helps scientists test predictive theories.

The researchers say the study’s findings will have important implications for future observations with upcoming space-borne gravitational-wave detectors, such as the Laser Interferometer Space Antenna (LISA), which is scheduled to launch in 2035 and is expected to detect similar supermassive black hole mergers across the universe.