Our galaxy’s supermassive black hole’s entourage of smaller black holes has been spotted for the first time.
Chuck Hailey from Columbia University and colleagues teased out the faint signal of a dozen “black hole binaries” – where a black hole and another object, such as a star, orbit each other – close to their supermassive counterpart in the centre of the Milky Way.
This cosmic posse, which could comprise more than 10,000 black holes in its entirety, was reported in Nature Astronomy today.
Black holes tend to come in two sizes. There are stellar-mass black holes – the super-dense remnant left when a star more than eight times the mass of the Sun dies.
But they have monster cousins.
These supermassive black holes, millions of times more massive than the Sun, are thought to be found in the centre of pretty much all large galaxies.
The Milky Way has one. It’s about 4 million times the mass of the Sun and resides in the Sagittarius A* region. (The asterisk denotes the source of radio waves thought to be generated by the supermassive black hole.)
And where you find a supermassive black hole, astrophysicists predicted, you should also see clusters of stellar-mass black holes surrounding it, producing what’s called a “density cusp”.
While black holes can form quite far from the centre of their galaxies, they gradually lose energy and start to migrate towards the galactic centre, Professor Hailey said, “much as sediment dumped in water will sink down under the influence of gravity.
“So [stellar-mass black holes] sink right down near the supermassive black hole … at the centre of the galaxy, where they are trapped by its gravity.”
Another theory is that massive stars, born in gas and dust that surrounds a supermassive black hole, implode to form black holes.
Either way, our galaxy’s supermassive black hole should be surrounded by hundreds of stellar-mass black holes – at least.
The problem has been spotting them.
So how do you ‘see’ a black hole?
One way to find the elusive objects is to look for bursts of X-rays that are produced when a black hole feeds off a companion star. The orbiting pair is called a black hole binary.
Despite telescopes being trained on the galactic centre for more than a decade, astrophysicists have had no joy.
“These outbursts are believed to be very rare, and so it appears we were unlucky,” Professor Hailey said.
So he and his colleagues decided not to seek these rare X-ray flashes.
Instead, they sniffed out the steady X-ray emissions expected from the disc of gas and dust that swirls around a black hole.
This was no easy feat. The centre of the Milky Way is around 25,000 light-years from Earth.
It’s also crowded with other X-ray-emitting objects, such as binary pairs that contain a white dwarf star.
Their trick, Professor Hailey said, was to look for lower-energy X-rays – a signature of a black hole binary – in the midst of higher-energy X-rays, which are a hallmark of white dwarf binaries.
“The analogy to light is colour,” he said. “The white dwarf binaries look bluer and the black hole binaries redder.”
Using data collected by the Chandra X-ray Observatory telescope, they found 12 black hole binaries within around 3.3 light-years from Sagittarius A*.
This, Professor Hailey said, “is just the tip of the iceberg of all the black holes there”.
Extrapolating from their distribution, he and his team calculated that there is probably hundreds of black hole binaries keeping Sagittarius A* company – and potentially more than 10,000 solo black holes.