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Trio of monster black holes rumble spacetime


Helical jets from one supermassive black hole caused by a very closely orbiting companion (see blue dots). The third black hole is part of the system, but farther away and therefore emits relatively straight jets. (ROGER DEANE/NASA GODDARD)

For the first time, three supermassive black holes have been discovered in a tight orbital dance inside the center of a galaxy 4 billion light-years away.

The discovery was made by radio telescopes located in Europe, Asia and South Africa, and astronomers believe that it’s extreme gravitational environments such as these that rumble spacetime, generating gravitational waves that are theorized to propagate throughout the cosmos.

“What remains extraordinary to me is that these black holes, which are at the very extreme of Einstein’s Theory of General Relativity, are orbiting one another at 300 times the speed of sound on Earth,” said Roger Deane, of the University of Cape Town, South Africa, in a press release. “Not only that, but 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.”

Two of the black holes are orbiting very close to one another, creating corkscrew-like jets of emissions from one of the black holes as they interact. The third black hole has a wider orbit and emits straight jets from its poles that aren’t impacted significantly by the other pair of black holes.

The observation was made possible by a global network of radio antennae that operate as one, vast array. The technique of linking radio telescopes on different continents and separated by up to 10,000 kilometers is known as Very Long Baseline Interferometry (VLBI) and, when linked, the observations can reveal detail in cosmological targets 50 times finer than the Hubble Space Telescope is capable of.

For this observation of the triple-black hole system, astronomers used data from the European VLBI Network (EVN) and correlated it at the Joint Institute for VLBI in Europe (JIVE) in Dwingeloo, the Netherlands.

Supermassive black holes are massive objects, ‘weighing-in’ at between 1 million to 10 billion times the mass of our sun. The majority of galaxies are known to contain these objects at their cores and are thought to have a key impact on galactic evolution and star formation. When galaxies merge, it is thought that the central black holes spiral in toward one another, eventually merging themselves.

It is therefore of paramount importance that astronomers study and understand supermassive black holes, so finding a triple system of supermassive black holes in tight orbits provide a privileged view into the life-cycle of these fascinating objects. And radio telescopes are the perfect tool for getting an up-close view.

“VLBI is widely recognized as one of the best ways to confirm close-pair black hole systems, but the main difficulty has always been pre-selecting the most promising candidates,” said JIVE scientist Zsolt Paragi. “Our research shows that close-pair black holes may be much more common than previously thought, although their detection require extremely sensitive and high-resolution observations.”

The researchers point out that next-generation radio telescopes, such as the Square Kilometer Array (SKA) that will be located in South Africa and Australia, will be perfect for further campaigns focused on compact black hole systems.

“We have always argued that next generation radio telescopes such as the SKA should operate in VLBI mode as well, jointly with existing radio telescope arrays,” added Paragi. “This will allow to broaden our understanding of how black holes grew and evolved together with their host galaxies.”

“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 Kilometer Array (SKA),” said Deane.

Although gravitational waves have been in the news a lot lately, astrophysicists predict that extreme environments such as these are powerful gravitational wave sources, as predicted by Einstein’s general relativity. So as sensitive detectors — such as the Laser Interferometer Gravitational Wave Observatory (LIGO) — attempt to track down these minute spacetime ripples, it is paramount that radio observations of orbiting supermassive black holes are carried out, characterizing the possible gravitational wave signatures that could be generated.

This research has been published June 25 in the journal Nature.