The Galactic Center Is Likely Full Of Black Holes

Composite X-ray/Radio image of the center of the Milky Way. Sagittarius A* is the bright dot. X-ray: NASA/CXC/UCLA/Z.Li et al; Radio: NRAO/VLA

Astronomers have discovered a dozen black holes orbiting within the central 3 light-years of the galaxy – tiny companions to Sagittarius A*, the supermassive black hole that dominates the core of the Milky Way. As reported in Nature, 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,” lead author Professor Charles Hailey, from Columbia University, told IFLScience. “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.”

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-rays binaries, a fraction of all the black holes near the center.

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."

Sagittarius A* weighs over 4 million times our Sun, but being incredibly dense, it has a tiny volume. Stellar black holes are even smaller – some would be the size of a city. So even though thousands might be lurking in the central parsec, they are probably well spaced out.  

Artist's impression of Sagittarius A* surrounded by smaller black holes orbited by stars. Columbia University



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