WASHINGTON – Astronomers have uncovered evidence of a mid-sized black hole nestled within Omega Centauri, a cluster believed to be the remnant core of a small galaxy devoured by the Milky Way 8 to 10 billion years ago.
Researchers observed the peculiar motion of seven stars within Omega Centauri, providing strong indications of this elusive black hole. These black holes are more massive than those formed from the collapse of single stars but smaller than the supermassive black holes found at the centers of most galaxies.
Omega Centauri houses approximately 10 million stars, and the black hole at its core is at least 8,200 times the mass of our sun, according to the researchers. For comparison, the Milky Way’s central black hole, Sagittarius A*, has 4 million times the mass of the sun, and even larger black holes in other galaxies boast billions of solar masses.
“There has been a long debate whether intermediate-mass black holes exist in general, and specifically in Omega Centauri, and our detection might help to resolve that debate,” said Maximilian Häberle of the Max Planck Institute for Astronomy, lead author of the study published in Nature.
This black hole is about 17,700 light-years from Earth. It was detected through the gravitational influence it exerts on seven nearby stars, observed over two decades by the Hubble Space Telescope.
The smaller galaxy, once about 10% the size of the Milky Way, likely hosted a black hole that would have grown into a supermassive one if it had continued feeding on surrounding material. However, the galactic merger halted this growth, leaving the black hole in its current intermediate state.
“In this merger process, the galaxy lost all of its gas, and hence the growth of its central black hole got interrupted, leaving it in an intermediate-mass state,” explained co-author Nadine Neumayer of the Max Planck Institute for Astronomy.
The collision stripped most stars from the smaller galaxy, leaving behind Omega Centauri. These mid-sized black holes, difficult to detect due to their smaller influence compared to supermassive black holes, have been suspected in similar contexts before.
“The most likely scenario for the formation of the intermediate-mass black hole at the center of Omega Centauri is the collision and merging of very massive stars very early on during the formation of the star cluster,” Neumayer added. These collisions created massive stars that quickly evolved into black holes, which then merged into the intermediate-mass black hole.
Understanding these mid-sized black holes may unlock secrets about the formation of supermassive black holes. “Intermediate-mass black holes are likely very common, especially in the early evolution of the universe,” Neumayer said. “They are thought to be the seeds for supermassive black holes.”