Trio of black holes marks the spot for gravitational waves

An international team of astronomers, headed up by University of Cape Town postdoctoral fellow Roger Deane, has discovered three supermassive black holes, caught in a tight orbit four-billion light years away.

The research, published in scientific journal Nature on Wednesday, suggests that these black-hole systems might be more common than we thought, and might open up a new era of the search for gravitational waves. Black holes form when very massive stars collapse in on themselves, but the formation of supermassive black holes is not certain even though most – possibly all – galaxies are thought to have one at their centres.

They used a technique called VLBI – Very Long Based Interferometry – which involves multiple telescopes sometimes tens of thousands of kilometres apart, to look 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).”

One of these science questions is gravitational waves, something that Albert Einstein predicted in his theory of general relativity, which have not been observationally proved.

“These binary supermassive black holes [are also] the strongest source of gravitational waves,” says Deane. 

The gravity of black holes is so strong that not even light can escape, hence the name. 

“The SKA and the MeerKAT are sensitive enough to be able to study the source of those gravitational waves. In high resolution.”

Deane said this is why the African VLBI Network will be an important addition to radio astronomy. Old telecommunications dishes are being converted into radio telescopes in Ghana, Kenya and Mozambique. A VLBI network requires at least four radio telescopes looking at the same object. South Africa’s Hartebeeshoek Radio Astronomy Observatory, for decades, was the only radio telescope on the African continent, and was part of the European VLBI.

“The further apart the dishes are, the finer the details,” said Deane, adding that dishes used for VLBI in Africa would dramatically improve our ability to “see” finer details.

Research co-author Zsolt Paragi from the Joint Institute for VLBI in Europe said: “VLBI is widely recognised 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.

“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.”

Paragi said that the next generation of telescopes “will allow us to broaden our understanding of how black holes grew and evolved together with their host galaxies”.

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