Trio of supermassive black holes surprises astronomers, and may provide hints into the exotic physics of gravitational waves

Friday, June 27, 2014, 1:03 PM - Gazing out into space, at the core of a distant galaxy that is radiating out incredible amounts of energy, a team of scientists discovered something incredible - three supermassive black holes locked in a whirling, spacetime-bending dance.

Astronomers have made some amazing discoveries about our universe, but one of the biggest was that each galaxy we can see, even including our own, has a black hole at its core - a sinkhole in spacetime so large that they tacked on the word 'supermassive' to describe them. Some of these supermassive black holes - like Sagittarius A*, the one at the core of the Milky Way - are quiet, docile and alone. Others are quiet, but are paired up, or are even part of a trio, orbiting around each other in a dance that is thought to eventually end in a merger of the black holes and a sudden end to the quiet. Others still are quite noisy, having been called 'quasars' (short for 'quasi-stellar radio source') even before we knew what they were, because they are very active, spewing out overwhelming amounts of energy due to gobbling up matter that's spiraling into them, and these have been known to exist in pairs and even triplets as well.

An international team of astronomers, led by Dr. Roger Deane of the University of Cape Town, was on the look out for binary quasars and focused on one known as SDSS J150243.091111557.3, at the core of a galaxy some four billion light years away from us. This quasar had been flagged as a potential binary already, since the spectrum of the energy it was beaming out had a split peak (a very good sign you're dealing with two objects, both emitting similar energy). To get an exceptionally fine look at these black holes during their dance, Deane and his team combining several different radio telescopes around the world into one huge telescope that measured at least 10,000 kilometres long, which would allow them to gather details so fine and crisp that they would make even the Hubble Space Telescope envious. When they pointed this expansive radio telescope at SDSS J150243.091111557.3, a surprise jumped out at them: one of the 'pair' of black holes at the core of this quasar was actually a pair in itself! So, rather than a pair, they were actually looking at a trio, with two of them forming the closest black hole binary pair ever found in one of these black hole triplets.

Credit: Roger Deane

"What remains extraordinary to me is that these black holes, which are at the very extreme of Einstein's General Theory of Relativity, are orbiting one another at 300 times the speed of sound on Earth," Deane said in a University of Cape Town news release. That's over 100 kilometres per second, or over three times the speed that Earth orbits around the Sun. That's pretty remarkable, considering that these two are still over 450 light years apart from one another. For reference, the closest star to our Sun, Proxima Centauri, is only 4.2 light years away.

As these two closely-orbiting black holes circle around each other, they emit helical jets of radiation that spiral out into space, while the third black hole, which is much more distant, emits its jets in a straight line as it orbits around the other two.

"Another very exciting aspect of this discovery is the fact that twisted jets are also observed in binary systems one million times smaller in our own Galaxy," said co-author Dr. Mickaël Coriat, also from the University of Cape Town, in the news release. "The main difference being that the pair of objects in that case is a small black hole (roughly the mass of our sun) and a normal star. This clearly demonstrates the universality of the law of physics on vastly different scales."

With the extreme nature of this spacetime-bending pair, as they push the limits of the laws of physics, they may also grant astronomers and astrophysicists new insight into gravitational waves. 

"This discovery not only suggests that close-pair black hole systems are much more common than previously expected," co-author Matt Jarvis, a professor at the University of Oxford and the University of the Western Cape, said in the news release, "but also predicts that radio telescopes such as MeerKAT and the African VLBI Network (AVN, a network of antennas across the continent) will directly assist in the detection and understanding of the gravitational wave signal. Further in the future the SKA (Square Kilometer Array) 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."

The research on this remarkable black hole trio was published this week in the journal Nature (click here).