Thousands of black holes may be hiding in Milky Way's centre

New York: A team of US astrophysicists have discovered a dozen black holes gathered around Sagittarius A* (Sgr A*) -- the supermassive black hole in the centre of the Milky Way Galaxy.

Scientists have done extensive fruitless searches for black holes around Sgr A* -- the closest supermassive black holes (SMBHs) to Earth and therefore the easiest to study.

"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," said lead author Chuck Hailey, Astrophysicist at Columbia University, US.

But, "there hasn't been much credible evidence", Hailey said.

In the study, appearing in the journal Nature, Hailey 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.

While most of the trapped black holes remain isolated, some capture and bind to a passing star, forming a stellar binary.

Researchers believe 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.

In the past, failed attempts to find evidence of such a cusp have focused on looking for the bright burst of X-ray glow that sometimes occurs in black hole binaries.

But, in the new study, the team searched for X-ray signatures of black hole-low mass binaries in their inactive state and were able to find 12 within three light years, of Sgr A*.

The researchers analysed the properties and spatial distribution of the identified binary systems and extrapolated from their observations that there must be anywhere from 300 to 500 black hole-low mass binaries and about 10,000 isolated black holes in the area surrounding Sgr A*.

"Knowing the number of black holes in the centre of a typical galaxy can help in better predicting how many gravitational wave events may be associated with them. All the information astrophysicists need is at the centre of the galaxy," Hailey said.