A team of astrophysicists led by Columbia University (USA) has discovered a dozen binary black holes around Sagittarius A * (Sgr A *), the supermassive black hole that resides in the center of the Milky Way. These observations confirm a theoretical prediction that there should be tens of thousands of invisible black holes at the center of galaxies. They are the first to support a decades-old prediction, opening up myriad opportunities to better understand the universe.
“Anything you want to learn about how large black holes interact with small black holes, you can learn by studying this distribution,” said Chuck Hailey, lead author of the study published in the journal Nature. “ The Milky Way is really the only galaxy we have where we can study how supermassive black holes interact with small ones because we cannot see their interactions in other galaxies. In a sense, this is the only laboratory we have to study this phenomenon.”
For more than two decades, researchers have unsuccessfully searched for evidence to support the theory that thousands of black holes surround supermassive black holes at the center of large galaxies. Until now, there hasn’t been much credible evidence about it.
Sgr A is the closest supermassive black hole to Earth and therefore the easiest to study. It is located 26,000 light years from Earth and 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 become black holes. Furthermore, black holes outside the halo are believed to fall under the influence of this supermassive black hole as they lose their energy, causing them to approach it and remain captive by its force.
While most of the trapped black holes remain isolated, some may eventually join a star in a stellar binary.
The invisible black holes
Researchers believe that there is a large concentration of these isolated and paired black holes in the Galactic Center, forming a density cusp that fills more and more as the distance to the supermassive black hole decreases.
Considering that black holes are isolated and do not emit any light, they are very difficult to detect. How have they been detected? Thanks to a new form of X-ray observation that can be captured from Earth.
To detect binary black holes, experts realized that they would have to look for the weakest but most stable X-rays emitted when the binaries are inactive.
“ Isolated and unbound black holes are just black, they don’t do anything. So looking for isolated black holes is also not a smart way to find them. But when black holes mate with a low-mass star, the ensemble emits bursts of X-rays that are fainter, but constant and detectable. If we could find black holes that are coupled with low-mass stars and we know what fraction of black holes will bond with low-mass stars, we could scientifically infer the population of isolated black holes there “Hailey clarifies.
The researchers drew on data captured by the Chandra X-ray space telescope to test this theory, looking for the characteristic fingerprints of black holes paired with low-mass stars. They finally found 12 of these binary systems at a distance of three light years from Sgr A *. After analyzing the properties and distribution of these objects, they stipulated that there must be between 300 and 500 low-mass binary black holes and approximately 10,000 isolated black holes in the area surrounding Sgr A *.
“This finding confirms an important theory and the implications are many,” Hailey said. “It will make significant progress in gravitational wave research, because knowing the number of black holes in the center of a typical galaxy can help more accurately predict how many gravitational wave events may be associated with them. All the information that astrophysicists need is there. in the center of the galaxy “, concludes the astrophysicist.
Referencia: Charles J. Hailey et al, A density cusp of quiescent X-ray binaries in the central parsec of the Galaxy, Nature (2018). DOI: 10.1038/nature25029
