Astronomers have scanned a cluster of stars that is the apparent remnant of a relatively small galaxy that was swallowed up by a giant galaxy 8 to 10 billion years ago. They are excited about what lies at the center of this cluster.
The unusual motion of seven stars in the cluster provides strong evidence for the presence of an elusive medium-sized black hole at its center, researchers said Wednesday. These are larger than the typical black holes formed by a single star explosion, but smaller than the supermassive black holes at the center of most galaxies.
This cluster, called Omega Centauri, contains about 10 million stars. Researchers say the black hole inside it is at least 8,200 times larger than our sun.
The mass of the supermassive black hole at the center of the galaxy, named Sagittarius A*, is 4 million times that of the Sun. And it is billions of times greater than the mass of supermassive black holes in other galaxies.
“There has been a long-standing debate about whether intermediate-mass black holes exist in general and in Omega Centauri in particular, and our discovery could help resolve this debate,” said astronomer Maximilian Haberle of the Max Planck Institute for Astronomy in Germany and lead author of the study published in the journal Nature.
This black hole is located about 17,700 light-years from Earth. A light-year is the distance light travels in one year, 5.9 trillion miles (9.5 trillion km). The only larger known black hole in the Milky Way is Sagittarius A*, which is located about 26,700 light-years from Earth.
Black holes are extraordinarily dense objects whose gravity is so strong that even light cannot escape them, making them difficult to detect. The black hole was detected based on how its gravitational pull affects the motion of seven fast-moving stars around it, recorded over two decades of observations by the Hubble Space Telescope.
Researchers believe the smaller galaxy, perhaps 10% of the size of the Milky Way, contained a black hole that, if left undisturbed, would have become supermassive as it fed on gas and other nearby material pulled in by its gravitational force. But the galaxy merger, which occurred when the Milky Way was about a quarter or a third of its current age, frozen the black hole in time.
“In this merger process the galaxy lost all its gas, and therefore the growth of its central black hole was inhibited, leaving it in an intermediate mass state,” said Nadine Neumayr, an astronomer at the Max Planck Institute for Astronomy and co-author of the study.
As a result of this merger, most of the stars in the smaller galaxy were ejected, leaving only the central cluster – now the Omega Centauri star cluster.
“Intermediate-mass black holes have also been found at the centers of low-mass galaxies or at the centers of some star clusters. However, detecting them has been very challenging. Due to their low mass relative to supermassive black holes, their sphere of influence is small,” Heberle said.
Other candidates for medium-sized black holes have been identified in previous research.
Black holes, which are comparable to the mass of a star, form when massive stars explode at the end of their life cycles and their cores collapse in on themselves.
“The most likely scenario for the formation of the intermediate-mass black hole at the center of Omega Centauri is the collision and merger of very massive stars very early in the star cluster’s formation. These stars come very close to each other, collide and form even more massive stars that grow into black holes quite quickly. The intermediate-mass black hole may develop through the merger of several of these black holes,” Neumayer said.
These medium-sized black holes may hold the key to understanding the formation of supermassive black holes.
“Intermediate-mass black holes are very common, especially in the early evolution of the universe,” Neumayer said. “They are thought to be the seeds of supermassive black holes.”
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