A team of international researchers got a bit of a shock recently when a supermassive black hole — something that normally anchors the centre of a galaxy — was spotted speeding away from its home.
The reason? Gravitational waves, says the research team.
"When you see a supermassive black hole that moves at 2,000 kilometres per second, and it's flying out of the centre of a galaxy, you have to try and understand the reason for this motion," says team leader Marco Chiaberge, of the Space Telescope Science Institute and Johns Hopkins University, in Baltimore.
"You need a huge amount of energy to displace a black hole like that," he says.
Which makes sense, considering how big it is.
A billion times the size of the sun, this rogue black hole is the biggest one kicked out of its home at the centre of a galaxy that researchers have seen — in this case, thanks to NASA's Hubble Space Telescope.
The images captured by the Hubble telescope showed a bright quasar — which indicates a black hole, the source of their energy — far from the heart of the galaxy.
The team, which also included University of Manitoba physics and astronomy professor Stefi Baum, calculated that it took the equivalent energy of 100 million supernovas exploding simultaneously to "kick" the black hole.
"What we think happened is, when two black holes collide, they get closer to each other — and before they collide, they start emitting gravitational waves," says Chiaberge.
How do gravitational waves eject a black hole from the centre of a galaxy?
- First comes the merger of two galaxies, each with a central black hole.
- The two black holes in the newly merged galaxy settle into the middle and begin whirling around each other, which produces gravitational waves.
- Over time, they move closer together, and finally merge.
- The energy from that merger propels the black hole away from the centre of the galaxy in the opposite direction of the strongest gravitational waves.
Since the two black holes aren't exactly identical, the waves go in a certain direction, and the merged black hole gets a recoil, and gets kicked out.
It's all because gravitational waves — the ripples in space first predicted by Albert Einstein — carry a huge amount of energy.
Chiaberge likens it to throwing a stone in a pond, which creates ripples in the water.
1st time 'this has ever been observed'
Black hole expert Harald Pfeiffer, from the University of Toronto's Canadian Institute for Theoretical Astrophysics, says the observations are "very, very unusual" and praised the research team for their peer-reviewed paper, coming out in the March 30 issue of the journal Astronomy & Astrophysics.
"It's the first time this has ever been observed," says Pfeiffer, who was not involved in the research.
He says it's striking that supermassive black holes can get such large "kicks" when they collide, which raises questions about how common this is throughout the universe, and whether or not researchers fully understand how galaxies collide and merge.
"The question here is, if this black hole actually makes it away from its galaxy, then you have a massive galaxy without a black hole in the middle?" he says.
Our own Milky Way galaxy, he notes, has a black hole at its centre — one that's four million times as massive as the sun.
Moving at 7 million km/h
As for the runaway black hole observed by Chiaberge's team, it's speeding away from that galaxy's centre at more than seven million km/h. The team calculated that it could travel from the Earth to the moon in three minutes, and could escape its galaxy in 20 million years.
But on Earth, there's no cause for concern: It's not coming anywhere near us.
The quasar that was spotted by the Hubble telescope, 3C 186, is from a host galaxy that's eight billion light-years away.
"There's so much empty space that the probability of it hitting anything is essentially zero," says Pfeiffer.
"Not only we are safe, but probably all the aliens in that other galaxy are also quite safe."