Two top South African scientists formed part of a team of global scientists who recently unveiled the first image that confirms the existence of the supermassive black hole at the centre of our own Milky Way galaxy.
Professor Roger Deane, an Extraordinary Professor at the University of Pretoria and Director for the Centre of Astrophysics at the University of Witwatersrand and Wits Postdoctoral Fellow, Dr Iniyan Natarajan, were part of this global collaboration with the Event Horizon Telescope team. They are the only two African-based representatives who were part of more than 300 of astronomers. The EHT team’s results have recently been published in a special issue of The Astrophysical Journal Letters.
The role of the South Africans
Professor Deane and Dr Natarajan’s contribution included precision measurements of the black hole ring size using a suite of algorithms. In addition, they also helped develop the sophisticated software suite used to simulate realistic EHT datasets. These were critical in order to compare theoretical predictions with the observations, and in turn to test theories of gravity.
Not only have the results of the EHT team provided overwhelming evidence that the image is indeed that of a black hole but it also offers valuable clues about how such giants, which are believed to reside at the centre of most galaxies, operate. The image was produced by a global research team called the Event Horizon Telescope (EHT) Collaboration, which uses observations from a worldwide network of radio telescopes. Scientists had previously tracked stars orbiting around something invisible, compact and very massive at the centre of the Milky Way. It is believed that this object – known as Sagittarius A (Sgr A, pronounced “sadge-ay-star”) – is a black hole and these latest images provide conclusive scientific evidence.
Even though the black hole cannot be seen with the naked eye, scientists generally explain it as being completely dark central region (called a “shadow”) surrounded by a bright ring-like structure. The new view captures light bent by the powerful gravity of the black hole, which is four million times more massive than our sun.
Said EHT Project Scientist Geoffrey Bower from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei: “These unprecedented observations have greatly improved our understanding of what happens at the very centre of our galaxy, and offer new insights on how these giant black holes interact with their surroundings.” He added: “We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity.”
According to the scientists, the black hole is about 27,000 light-years away from Earth. To image it, the team created the powerful EHT, which linked together eight existing radio observatories across the planet to form a single “Earth-sized” virtual telescope. The EHT observed Sgr A on multiple nights, collecting data for many hours in a row, similar to using a long exposure time on a camera.
“Southern Africa holds a distinct geographic advantage to host new EHT telescopes, especially to make movies of the Milky Way’s supermassive black hole, which lies directly above us in the southern sky. A campaign to add these game-changing African nodes to the global network is underway with several national and international partners, including Wits and the University of Pretoria,” said Professor Deane. He said this will give impetus to the future African expansion of the Square Kilometre Array mid-frequency array centred in the Karoo National Park, with the South African Radio Astronomy Observatory’s MeerKAT telescope serving as a precursor”.
Similarities between two black holes
In 2019 EHT collaboration yielded the first image of a black hole, called M87, at the centre of the more distant Messier 87 galaxy. The two black holes look remarkably similar, even though our galaxy’s black hole is more than a thousand times smaller and less massive than M87. Scientists say this achievement was considerably more difficult than for M87, even though Sgr A is much closer to us.
Explained EHT’s scientist Chi-kwan (‘CK’) Chan, from Steward Observatory and Department of Astronomy and the Data Science Institute of the University of Arizona, US: “The gas in the vicinity of the black holes moves at the same speed – nearly as fast as light – around both Sgr A and M87. But where gas takes days to weeks to orbit the larger M87, in the much smaller Sgr A it completes an orbit in mere minutes. This means the brightness and pattern of the gas around Sgr A was changing rapidly as the EHT Collaboration was observing it – a bit like trying to take a clear picture of a puppy quickly chasing its tail.”
The use of supercomputers
The researchers had to develop sophisticated new tools that accounted for the gas movement around Sgr A. The breakthrough saw more than 300 researchers from 80 institutes work together as part of the EHT Collaboration. The team worked rigorously for five years, using supercomputers to combine and analyse their data, all while compiling an unprecedented library of simulated black holes to compare with the observations.