Study provides clues to the origin of a supermassive black hole in the center of a galaxy

The origin of the aptly named supermassive black holes – which can be more than a million times as massive as the Sun and are located at the center of most galaxies – remains one of the great mysteries of the cosmos.

Now researchers from the Nevada Center for Astrophysics at UNLV (NCfA) have discovered compelling evidence that the supermassive black hole at the center of our Milky Way, known as Sagittarius A* (Sgr A*), is likely the result of an earlier cosmic merger.

The study was published on September 6 in the journal Natural astronomybuilds on recent observations from the Event Horizon Telescope (EHT), which took the first direct image of Sgr A* in 2022. The EHT, the result of a global research collaboration, synchronizes data from eight existing radio observatories around the world to create a giant virtual telescope the size of Earth.

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

The UNLV team examined various growth models to understand the unusually rapid rotation and misalignment of Sgr A* relative to the Milky Way’s angular momentum. The team showed that these unusual properties are best explained by a large merger event between Sgr A* and another supermassive black hole, likely from a satellite galaxy.

“This discovery paves the way for our understanding of how supermassive black holes grow and evolve,” said Wang, the study’s lead author and an NCfA postdoctoral fellow at UNLV. “The misaligned high rotation of Sgr A* suggests that it may have merged with another black hole, dramatically changing its rotation amplitude and direction.”

Using sophisticated simulations, the researchers modelled the effects of a merger, considering various scenarios consistent with the observed spin properties of Sgr A*. Their results suggest 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 was able to merge with the Gaia-Enceladus galaxy,” said Zhang, a distinguished professor of physics and astronomy at UNLV 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* is located at the center of the galaxy, more than 27,000 light-years from Earth, and sophisticated tools like the EHT provide direct images that help scientists test predictive theories.

According to the researchers, the findings from the study will have significant implications for future observations with future space-based gravitational wave detectors, such as the Laser Interferometer Space Antenna (LISA), which is scheduled to launch in 2035 and is expected to detect similar mergers of supermassive black holes throughout the universe.