Scientists have previously predicted that the supermassive black holes near the centres of large galaxies are surrounded by clusters of stellar-mass black holes. However, previous searches of the Galactic Centre, where the nearest supermassive black hole Sagittarius A* (Sgr A*) resides, have been unsuccessful. The unsuccessful attempts have focused on looking for the bright burst of x-ray glow that sometimes occurs in black hole binaries. Using archival data from the Chandra X-ray Observatory telescope, a Columbia University-led team of astrophysicists has discovered hundreds of inactive low-mass x-ray binary systems—binary star systems in which one of the components is a black hole—within a parsec (approximately 3.3 light years) of Sgr A*, in the centre of the Milky Way Galaxy. The researchers suggest that the properties and spatial distribution of these x-ray binary systems point to a total population of hundreds of black holes associated with binary systems within a parsec of the centre, and many more isolated black holes. The finding is the first to support a decades-old prediction, opening up myriad opportunities to better understand the universe, according to the study, which was published in the journal Nature. For more than two decades, researchers have searched unsuccessfully for evidence to support a theory that thousands of black holes surround supermassive black holes (SMBHs) at the centre of large galaxies. Columbia astrophysicist Chuck Hailey, co-director of the Columbia Astrophysics Lab and lead author of the study, said, "everything you'd ever want to learn about the way big black holes interact with little black holes, you can learn by studying this distribution." He also illustrated that the Milky Way is really the only galaxy we have where we can study how supermassive black holes interact with little ones "because we simply cannot see their interactions in other galaxies. In a sense, this is the only laboratory we have to study this phenomenon." "There are only about five dozen known black holes in the entire galaxy—100,000 light years wide—and there are supposed to be 10,000 to 20,000 of these things in a region just six light years wide that no one has been able to find," Hailey said, adding that extensive unsuccessful studies have been made for black holes around Sgr A*, the closest SMBH to Earth and therefore the easiest to study. "There hasn't been much credible evidence," he added. The lead author also explained that Sgr A* is surrounded by a halo of gas and dust that provides the perfect breeding ground for the birth of massive stars, which live, die, and could turn into black holes there. Additionally, black holes from outside the halo are believed to fall under the influence of the SMBH as they lose their energy, causing them to be pulled into the vicinity of the SMBH, where they are held captive by its force. While most of the trapped black holes remain isolated, some capture and bind to a passing star, forming a stellar binary. Researchers believe that there is a heavy concentration of these isolated and mated black holes in the Galactic Centre, forming a density cusp, which gets more crowded as distance to the SMBH decreases.
