Multiple Black Holes Discovered

The galactic center is likely populated
with thousands of black holes

Astronomers have discovered multiple black holes orbiting within the center of the galaxy, approximately 3 light-years in diameter. Scientists have long suspected that there might be a host of smaller black holes which are tiny companions to Sagittarius A*, the supermassive black hole that dominates the core of the Milky Way. These observations confirm a theoretical prediction that there should be up to 20,000 stellar black holes at the very center of galaxies.

“The existence of a large population of black holes close to the supermassive black hole is a fundamental prediction of stellar dynamics that was made two decades ago,” said Professor Charles Hailey, from Columbia University. “But there hasn’t been any substantial evidence for the existence of these black holes. This discovery confirms a real prediction that we made about how black holes should be born and evolve and go into the galactic center.”

Black holes by their nature are difficult to see, but now scientists have been able to spot the traces sent out by the smaller black holes. The team used the X-ray space telescope, Chandra, to observe a special class of objects known as low-mass X-ray binaries, which form when a stellar black hole is orbited by a star that has a mass similar to our Sun. The team found only 12 sources, but their position and distribution suggest there could be hundreds of these X-ray binaries, a fraction of all the black holes near the center. The hunt came up with evidence of 300 to 500 of the binaries, from which it was possible to infer how many "isolated" black holes there must be at the galactic core. The answer is speculated to be about 20,000.

The team suggests that there are two possibilities for the formation of these black holes: They can either have formed further out from the supermassive black hole and then moved in or they could have formed directly from the disk of gas surrounding Sagittarius A*. Afterward, the black holes were able to capture a passing star, turning the system into a binary that can be seen in X-ray. These observations are expected to spark a lot of theoretical modeling to predict precisely what the full population of black holes might be like. The models will give information on what to expect and, in turn, every new sighting will help refine the model. That said, there might already be objects that have been detected but not recognized as black holes.

“There is more data available from another instrument on the Chandra observatory and we are very eager to look at data from that instrument," Professor Hailey added. "We believe that it may allow us to detect more of these black hole binaries. This will add even more information for theorists to use in their modeling.All the information astrophysicists need is at the center of the galaxy."

The supermassive black hole at the heart of the Milky Way known as Sagittarius A* weighs over 4 million times than our Sun, but being incredibly dense, it has a tiny volume. It is approximately 26,000 light years from Earth. Stellar black holes are much smaller with some being the size of a city. So even though thousands might be lurking in the central parsec, they are probably well spaced out. The halo of gas and dust around Sgr A is thought to provide the perfect breeding ground for massive stars that collapse into black holes when they die. These black holes, and others from outside the halo, are pulled towards Sgr A and held captive around it, scientists believe.