Sagittarius A’s Unusual Spin Linked to a Supermassive Black Hole Collision 9 Billion Years Ago
TL;DR
Astronomers investigating the supermassive black hole at the center of our galaxy, Sagittarius A*, have found strong evidence suggesting its unusual spin is the result of a violent merger with another supermassive black hole around 9 billion years ago. This discovery, using data from the Event Horizon Telescope, could explain why the black hole’s spin is misaligned with the Milky Way’s rotation. Mergers between supermassive black holes can shape galaxies and influence black hole growth. Future space-based gravitational wave telescopes like LISA will further explore these colossal cosmic collisions.
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Astronomers investigating the supermassive black hole at the center of the Milky Way have uncovered “strong evidence” that might finally shed light on its puzzling history. Sagittarius A*, situated 26,000 light-years away in the heart of our galaxy, is a massive distortion in space-time with a mass 4 million times greater than our sun and a width of 14.6 million miles (23.5 million kilometers).
However, how this black hole formed, and why it spins unusually fast and out of sync with the rest of the galaxy, remains uncertain. Now, new data from the telescope that captured the first image of this black hole in 2022 has revealed a vital clue: The Sagittarius A* we observe today was likely the product of a violent merger with another enormous black hole billions of years ago, and it’s still exhibiting the aftereffects of this catastrophic collision. The research team published these findings on Sept. 6 in the journal Nature Astronomy.
“This discovery is a breakthrough in understanding how supermassive black holes grow and change,” said Yihan Wang, lead author and astrophysicist at the University of Nevada, Las Vegas (UNLV), in a statement. “The unusual, misaligned high spin of Sgr A* suggests it may have collided with another black hole, dramatically changing its spin speed and direction.”
Even though it accounts for just 0.0003% of the Milky Way’s mass, Sagittarius A* is a formidable force that intermittently absorbs matter and then ejects it at nearly the speed of light, setting off a feedback process that has shaped our galaxy since its early days. Scientists believe that this enormous black hole began like others—formed by the collapse of a massive star or gas cloud, before devouring everything in its vicinity. After growing to an immense size, it could even absorb other supermassive black holes. Supermassive black hole mergers happen when entire galaxies merge. Irregularities in the Milky Way’s disk indicate that it has likely merged with at least a dozen other galaxies over the past 12 billion years. However, astronomers are still unsure how significant black hole mergers are in the formation of supermassive black holes, or if these space-time distortions can expand to such gigantic sizes solely by absorbing gas and dust.
To uncover direct evidence of Sagittarius A*’s origins, researchers from the new study used data from the Event Horizon Telescope to develop a model of the black hole’s behavior over time. In various simulations, astronomers found that the black hole’s strange spin — which is entirely misaligned with the Milky Way’s rotation — could be best explained by a massive merger event involving the supermassive black hole of another galaxy.
“This merger likely occurred around 9 billion years ago, following the Milky Way’s merger with the Gaia-Enceladus galaxy,” study co-author Bing Zhang, a professor of physics and astronomy at UNLV, said in the statement. This merger not only adds evidence to the idea that black holes can grow ever larger by gobbling up their own kind, but also “provides insights into the dynamic history of our galaxy,” Zhang added.
To uncover further evidence of giant black holes merging across the universe, the researchers say they are waiting for the construction of space-based gravitational wave telescopes such as NASA‘s and the ESA‘s Laser Interferometer Space Antenna (LISA) project. Once LISA is lifted to space in 2035, it will detect the shock waves created in space-time when supermassive black holes collide.
