New research shows that the Milky Way's central supermassive black hole was formed by merging with another black hole about 9 billion years ago. The discovery, using data from the Event Horizon Telescope, helps explain the black hole's rapid rotation and misalignment with the galaxy.

The origin of supermassive black holes - which weigh more than a million times the mass of the sun and lie at the centers of most galaxies - remains one of the greatest mysteries in the universe.

Now, researchers at the UNLV Nevada Center for Astrophysics (NCfA) have found compelling evidence that the supermassive black hole at the center of the Milky Way—Sagittarius A* (SgrA*)—was likely formed from a past cosmic merger.

The study builds on recent observations from the Event Horizon Telescope (EHT), which captured the first direct image of Sagittarius A* in 2022, recently published in the journal Nature Astronomy. The EHT is the result of a global research collaboration that synchronizes data from eight existing radio astronomy observatories around the world to create a giant, Earth-sized virtual telescope.

This is the first image of SgrA*, the supermassive black hole at the center of the Milky Way. This is the first direct visual evidence of the black hole's existence. It was captured by the Event Horizon Telescope (EHT), an array that links together eight existing radio observatories on Earth to form an "Earth-sized" virtual telescope. The telescope is named after the event horizon, the boundary of a black hole from which no light can escape. Source: EHT Collaborative Group

UNLV astrophysicists Yihan Wang and Bing Zhang used EHT observations of Sagittarius A* to look for evidence of how it might have formed. Supermassive black holes are thought to form through long periods of accretion of matter or the merger of two existing black holes.

The UNLV research team studied various growth models to understand SgrA*'s peculiar rapid rotation and misalignment relative to galactic angular momentum. The team demonstrated that a major merger event between SgrA* and another supermassive black hole, possibly from a satellite galaxy, best explains these unusual features.

"This discovery paves the way for our understanding of how supermassive black holes grow and evolve. SgrA*'s high spin misalignment suggests that it may have merged with another black hole, dramatically changing the amplitude and direction of its spin."

Through sophisticated simulations, the researchers modeled the effects of the merger, taking into account various scenarios consistent with the observed spin properties of SgrA*. The results show that mergers with a 4:1 mass ratio and a highly tilted orbital configuration can reproduce the spin properties observed by the EHT.

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

SgrA* is located at the center of the Milky Way, more than 27,000 light-years from Earth, and precision tools like the EHT can provide direct imaging to help scientists test predictive theories.

The researchers say the study's findings will have a major impact on future observations from upcoming 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 supermassive black hole mergers in the universe.

Compiled from/SciTechDaily