Binary black holes might be common, say experts

In a galaxy far, far away – four-billion light years away, to be precise – huge jets of matter spiral around a dark centre. This dark centre is a black hole, one of the most enigmatic objects in our universe: it has such a strong gravitational field that not even light can escape, meaning it can only be detected from how things behave around it. 

But a team of international astronomers, headed up by the University of Cape Town’s Dr Roger Deane, has found that in this dark centre there are actually three supermassive black holes, circling each other at 300 times the speed of sound.

Germany’s Max Planck Institute for Radio Astronomy, whose astronomers were part of the research, said: “This is the tightest trio of black holes known to date, and is remarkable since most galaxies have just one black hole – usually with a mass between one million to 10-billion times that of the sun at their centre.” The discovery also suggested that binary black holes might be more common than previously thought, it said.

Black holes form when massive stars collapse in on themselves, but the formation of supermassive black holes is not certain. Some scientists believe that they could be the result of the black hole sucking matter into it and growing, or through the merging of black holes. Most – possibly all – galaxies are thought to have a supermassive black hole at their centres. Understanding black holes could help us understand how galaxies form.

‘Black holes could grow’
“We have found the first needle in the ‘middle age’ universe and I hope that we will find much more and even closer systems of this kind in the near future,” says Hans-Rainer Klöckner from the Max Planck Institute and a co-author of the paper, which was published in journal Nature on Wednesday. “Such close binaries will not only show us how supermassive black holes could grow or how they could alter our space-time, they will also help us to understand the inner workings and the interplay between jets and the accretion disc surrounding black holes.”

“What remains extraordinary to me is that these black holes, which are at the very extreme of [Albert] Einstein’s theory of general relativity, are orbiting one another at 300 times the speed of sound on earth,” said Deane. Black holes are also thought to be a key to understanding gravitational waves which – Einstein hypothesised – dictate the way planets, stars and galaxies move.

They used a technique called VLBI, or very long based interferometry, which involves multiple telescopes, sometimes up up tens of thousands of kilometres apart, looking at the same object. 

“Using the combined signals from radio telescopes on four continents, we are able to observe this exotic system one-third of the way across the universe,” said Deane, who is a postdoctoral fellow. “It gives me great excitement as this is just scratching the surface of a long list of discoveries that will be made possible with the Square Kilometre Array [SKA].”

The SKA will be the largest radio telescope in the world and, similar to VLBI, which means that its thousands of antennas will be surveying the same objects. These antennas will dot the landscapes of Australia and South Africa, with satellite stations in the eight African partner countries.

“Further in the future the SKA will allow us to find and study these systems in exquisite detail, and really allow us gain a much better understanding of how black holes shape galaxies over the history of the universe,” said Mike Jarvis, from the Universities of Oxford and the Western Cape.

Sarah Wild 1
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