A small satellite galaxy of the Milky Way, known as Segue 1, contains a supermassive black hole far larger than expected for its size. This discovery challenges long-held assumptions that such dwarf galaxies are primarily held together by dark matter. Researchers used computer models to reveal the black hole's presence at the galaxy's center.
Segue 1, one of the Milky Way's smallest galactic neighbors, has just about 1,000 stars—dwarfed by the Milky Way's hundreds of billions. Long thought to be dominated by dark matter due to insufficient stellar gravity to bind it, the galaxy now appears to host a supermassive black hole at its core. This black hole is roughly 10 times more massive than all of Segue 1's stars combined, with a mass equivalent to about 450,000 suns.
Nathaniel Lujan at the University of Texas at San Antonio and his colleagues ran extensive computer simulations to model Segue 1's dynamics. They initially expected dark matter to be the key factor but found otherwise. “I was running hundreds of thousands of models, and I wasn’t finding anything that fit,” Lujan said. “And then finally I decided to mess with the black hole mass and all of a sudden it started to work.”
The galaxy's stars suggest it formed around 400 million years after the universe's first stars, leaving little time for such a large black hole to grow, especially with the Milky Way drawing away nearby gas. This raises questions about black hole formation in the early universe.
“This probably means that there are more supermassive black holes than we thought,” Lujan noted. These could explain some gravitational effects previously ascribed to dark matter. However, it remains unclear if Segue 1 represents other dwarf galaxies, prompting further searches. The findings are published in The Astrophysical Journal Letters (DOI: 10.3847/2041-8213/ae0b4f).