Multiple black holes punctuate the middle of the Milky Way

The centre of the Milky Way is packed with black holes like cosmic Swiss cheese, astronomers say, with new research confirming 50-year-old predictions.

About 26,000 light-years from Earth, a monstrous black hole lurks like a spider at the centre of a great gravitational web, with our entire galaxy rotating around it. This supermassive structure, called Sagittarius A*, is four million times more massive than our sun. 

Astronomers have long predicted that it is surrounded by as many as 20,000 smaller black holes, formed from the collapse of massive stars, but they have never been observed — until now.

For the first time, an international team of astronomers led by astrophysicist Charles Hailey from Columbia University in New York, US, has found have found a dozen of these stellar mass black holes within 3.3 light-years (about 30 trillion kilometres) of Sagittarius A*. 

Their results, published in the journal Nature, were obtained by sifting through 12 years of archive data from the Chandra X-ray Observatory, which has been in orbit around the Earth since 1999. 

According to CSIRO astronomer Lisa Harvey-Smith, who was not involved in the study, we expect the Milky Way’s centre to be riddled with stellar mass black holes for two reasons. 

First, she says, because “there are large clusters of hot, massive stars towards the centre of our galaxy and these stars become black holes at the end of their lives.”

Second, interactions cause black holes that formed further out to lose energy and sink closer over time, until they are captured by Sagittarius A*’s colossal gravitational pull.

Astronomers haven’t detected this bonanza of black holes before because they aren’t exactly easy to spot. 

“Isolated black holes just don’t do much of anything,” says lead author Hailey. “They are very weak emitters of X-rays when they eat gas and dust. And otherwise, they are pretty silent.”

Astronomers have previously looked for black holes that are gravitationally bound to a companion star, which can provide fuel for very bright X-ray outbursts — but such eruptions are rare.

Instead, Hailey and his team chose to detect X-ray binary systems by carefully searching for faint, steady X-ray emissions. Their patience paid off when they found a dozen low-mass, inactive systems.

“Now we know the centre is teeming with black holes,” says Hailey. “What could be cooler than that?”

The distribution of these X-ray binary systems indicates that there may be hundreds more the same distance from Sagittarius A*, as well as many more isolated individual black holes.

“The observations can now fuel a lot more interesting work by theorists concerning exactly how these black hole binaries form and why they end up where we find them,” Hailey says. “Theory is a lot more interesting when you have observations to compare against.”

The discovery can also help refine our ideas of the bizarre, rare binary systems that produce gravitational waves. {%recommended 3781%}

And these may turn out to be the best way to detect and study black hole populations in the centres of other galaxies, too distant for us to search using X-ray emissions.

However, Harvey-Smith says that “with this sort of study we always need to exercise caution in how we interpret the data, since the way that objects are classified can lead to some ambiguities. There might be some non-black-hole objects such as unusual stars lumped in with the black hole sample.”

Indeed, the authors note that some of the objects may actually be millisecond pulsars — rotating neutron stars that emit flashes of radiation like lighthouses. Recent results indicate that these objects may also be concentrated near the heart of the Milky Way.

“However, as our instruments get better it will be possible to untangle these uncertainties and gain a better understanding of the centre of our galaxy,” says Harvey-Smith.