A trio of closely orbiting supermassive black holes has been spotted in a galaxy nearly 4.2 billion light-years away. The discovery was made by an international team of astronomers, which points out that such triple systems are very rare because most galaxies have just one black hole at their centre. This system is particularly interesting to astronomers because two of the three black holes are very closely bound, forming a "tight" binary pair within the system.

Astronomers know that supermassive black holes – the largest type of black hole, which can be billions of solar masses – lie at the heart of most galaxies, including our own Milky Way. Most galaxies are believed to evolve via collisions and mergers between smaller galaxies, so some of the larger galaxies should contain multiple supermassive black holes. Having two or more such gravitational powerhouses in a galaxy would have profound effects on its structure and dynamics. As a pair of supermassive black holes orbit one another, for example, the binary system's gravity would disrupt the gas and stars at the centre of the host galaxy. This, in turn, could lead to a burst of star formation or even the ejection of one of the black holes from the galaxy.

Heavyweight triplets

To date, only a few galaxies with two supermassive black holes have been found, and just four triple black-hole systems are currently known. The closest known spacing between black holes in a binary system is 2.4 kiloparsecs – about 1/10th the diameter of the main disc of the Milky Way. The new system, detected by Roger Deane of the University of Cape Town, South Africa, and colleagues, consists of two supermassive black holes separated by a mere 140 parsecs, while the third of the trio is 7 kiloparsecs from the close-knit pair. The two black holes in the pair are orbiting one another at high speed – more than 100,000 m s–1.

The team made its discovery while studying six galaxies that were thought to host binary supermassive black-hole systems based on near-infrared and optical observations. The researchers found that one of the black holes was actually two, and hence that particular system is a triple. Because the astronomers did not have to search through many candidates to find the system, they believe that tightly knit binaries and indeed triple systems of black holes could be more common than previously thought.

Giant radio telescope

The team employed a technique known as very long baseline interferometry (VLBI) to study the trio. VLBI creates a giant radio telescope spanning thousands of kilometres across the globe by combining the signals from large radio antennas that can be separated by up to 10,000 km. This allows astronomers to see detail 50 times finer than that possible with the Hubble Space Telescope. The current observations were done with the European VLBI Network (EVN) and the data were correlated at the Joint Institute for VLBI in Europe (JIVE) in the Netherlands.

Deane told physicsworld.com that the discovery demonstrates the power of VLBI to differentiate between multiple objects in systems that are huge distances from Earth. Before the latest discovery, a pair of supermassive black holes with the closest orbit (about 7 parsecs apart) was spotted in a galaxy some 750 million light-years from Earth. "Our system is 4.2 billion light-years away, which is much more distant than the closest known pair, demonstrating that the VLBI technique can be used to probe close black-hole pairs across a fair fraction of cosmic time," he says.

Spinning jets

The presence of the bound pair was also revealed via a much more prominent feature – the large-scale radio jets emanating from the black holes. Such astrophysical jets are a common feature of supermassive black holes – accreted matter collecting around the event horizon of the black hole is ejected along its axis of rotation as it tries to fall into the hole. The triple system has three such jets, and Deane and colleagues found that the presence of the tight pair is imprinted onto the properties of the jets. Indeed, the orbital motion of the black holes in the pair twists the jets into a helical or corkscrew-like "S" shape. This provides astronomers with a "smoking gun" for a binary black-hole system that could be used in future searches.

Deane also points out that this extreme triple system could be creating gravitational waves – ripples in the very fabric of space–time. Future telescopes, such as the Square Kilometre Array, should be able to detect these ripples for black holes that are even closer together. "It fills me with 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," Deane says.

The research is published in Nature.