Johannesburg - Astronomers have unveiled the first image of the supermassive black hole at the centre of the earth’s Milky Way galaxy, a discovery that was announced at simultaneous press conferences around the world.
This includes at the Wits Planetarium in Joburg as this discovery was made possible through the ingenuity of more than 300 researchers from 80 institutes around the world that together make up the EHT (Event Horizon Telescope) Collaboration.
Two of them are from Africa, including Wits post-doctoral fellow, Dr Iniyan Natarajan, and Professor Roger Deane, Director of the Wits Centre for Astrophysics and Extraordinary Professor at the University of Pretoria.
During the briefing, scientists explained that this result provides overwhelming evidence that the object is indeed a black hole and yields valuable clues about the workings of such giants, which are thought to reside at the centre of most galaxies.
The image is a long-anticipated look at the massive object that sits at the very centre of our galaxy and was produced by using observations from a worldwide network of radio telescopes.
Scientists had seen stars orbiting around something invisible, compact, and very massive at the centre of the Milky Way.
This strongly suggested that this object — known as Sagittarius A* (Sgr A*) — is a black hole, and today’s image provides the first direct visual evidence of it.
Experts argued that while humans are unable to see the black hole itself, because it is completely dark, glowing gas around it reveals a tell-tale signature: a 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.
“We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity,” says EHT project scientist Geoffrey Bower from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei.
He said the 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”.
Scientists have explained that because the black hole is about 27 000 light-years away from Earth, it appears to those on earth to have about the same size in the sky as a doughnut on the Moon.
In order 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 and the breakthrough follows the EHT collaboration’s 2019 release of the first image of a black hole, called M87*, at the centre of the more distant Messier 87 galaxy.
Sera Markoff, co-chair of the EHT Science Council and a professor of theoretical astrophysics at the University of Amsterdam, the Netherlands said that 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.
“We have two completely different types of galaxies and two very different black hole masses, but close to the edge of these black holes they look amazingly similar,” she said.
“This tells us that general relativity governs these objects up close, and any differences we see further away must be due to differences in the material that surrounds the black holes.”
EHT scientist Chi-kwan (‘CK’) Chan, from Steward Observatory and Department of Astronomy and the Data Science Institute of the University of Arizona added that the discovery of the black hole was considerably more difficult than for M87, even though Sgr A* is much closer to us.
“The gas in the vicinity of the black holes move 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 researchers had to develop sophisticated new tools that accounted for the gas movement around Sgr A* and while M87* was an easier, steadier target, with nearly all images looking the same, that was not the case for Sgr A*.
This is because the image of the Sgr A* black hole is an average of the different images the team extracted, finally revealing the giant lurking at the centre of our galaxy for the first time.
In addition to developing complex tools to overcome the challenges of imaging Sgr A*, the team worked vigorously 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.
The researcher’s contributions included precision measurements of the black hole ring size using a suite of algorithms, as well as developing the sophisticated software suite used to simulate realistic EHT datasets.
These were critical to robustly compare the observations with predictions from Einstein’s General Theory of Relativity.
Scientists are particularly excited to finally have images of two black holes of very different sizes, which offers the opportunity to understand how they compare and contrast.
They have also begun to use the new data to test theories and models of how gas behaves around supermassive black holes.
This process is not yet fully understood but is thought to play a key role in shaping the formation and evolution of galaxies.
“Now we can study the differences between these two supermassive black holes to gain valuable new clues about how this important process works,” EHT scientist Keiichi Asada from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei said.
“We have images for two black holes — one at the large end and one at the small end of supermassive black holes in the Universe — so we can go a lot further in testing how gravity behaves in these extreme environments than ever before.”
While progress on the EHT continues, a major observation campaign in March 2022 included more telescopes than ever before.
Wits’s Professor Deane explained that the ongoing expansion of the EHT network and significant technological upgrades will allow scientists to share even more impressive images as well as movies of black holes in the near future.
“Southern Africa holds a distinct geographic advantage to host new EHT telescopes, especially if we wish to make movies of the Milky Way’s supermassive black hole, which passes directly above us in the southern sky.”
Efforts to add these African nodes to the global network are underway with several national and international partners, including Wits and the University of Pretoria.
In addition to enabling higher precision tests of general relativity, the expansion of the EHT into Africa has a strong synergy with the future continental expansion of the Square Kilometre Array mid-frequency array centred in the Northern Cape, with the South African Radio Astronomy Observatory’s MeerKAT telescope serving as its precursor.