Astronomers have released the first image of the giant lurking at the centre of our galactic neighbourhood — a supermassive black hole about 27 000 light years from Earth.
On Thursday, they unveiled the groundbreaking image as the first direct visual evidence of Sagittarius A*, pronounced “sadge-ay-star”, the black hole at the dark heart of the Milky Way.
The breakthrough is the result of a global effort by more than 300 scientists from 80 institutions, including the University of the Witwatersrand, through the Event Horizon Telescope Collaboration (EHTC).
The international partnership captures images of black holes using a virtual earth-sized telescope named after the “event horizon”, which is the boundary of the black hole beyond which no light can escape. The image was produced by the EHTC, using observations from a worldwide network of radio telescopes.
Most studied astrophysical objects
The team’s results were published on Thursday in a special issue of The Astrophysical Journal Letters. In it, Geoffrey C Bower, the project scientist for the EHTC, described how Sagittarius A* was identified nearly 50 years ago as the nearest supermassive black hole candidate and ranks among the most studied astrophysical objects.
Sagittarius A* “is the ultimate laboratory for black hole astrophysics”, Bower said. “These results are the culmination of a multi-year effort by the EHTC and a decades-long journey by the astronomy community to approach the event horizon through high-resolution imaging.”
According to the EHTC, the image is a long-anticipated look at the “massive object that sits at the very centre of our galaxy”.
Scientists had previously seen stars orbiting around something invisible, compact and very big at the centre of the Milky Way. The image provides overwhelming evidence that the object is indeed a black hole.
The discovery, the scientists say, yields valuable clues about the workings of such giants thought to reside at the centre of most galaxies.
Although one cannot see the black hole itself, glowing gas around it shows 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.”
The breakthrough follows the EHTC’s April 2019 release of the first image of a black hole, called M87*, at the centre of the more distant Messier 87 galaxy. Sagittarius A* is more than a thousand times smaller 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,” Sera Markoff, co-chair of the EHT Science Council and a professor of theoretical astrophysics at the University of Amsterdam, the Netherlands, said in the statement. “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.”
‘Puppy chasing its tail’
The achievement was much more difficult than for M87*, even though Sagittarius A* is much closer to us, according to EHT scientist Chi-kwan Chan. “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.”
Professor Roger Deane, the director of the Wits Centre for Astrophysics and extraordinary professor at the University of Pretoria, explained that Sgr A* is a 1 000 times faster black hole than M87*.
“The ring changes within minutes, not months,” he said. “The analogy I’ve got is of a three-year-old child and a one-year-old child in a photograph together. Well, it doesn’t work. They move away constantly so try and do that in a very dark setting where you’re trying to do a time lapse long exposure, that’s the problem with Sgr A*. It’s not going to stay in one place. The image, the structure of the ring, changes while we’re observing it. We had to invent completely new techniques to actually do this.”
Peering through the Milky Way
Another challenge, Deane said, was that the team of researchers had to peer through the gas and dust of the Milky Way, which blurs the image. “If you think of a fried egg with the yolk in the middle, we’re out in suburbia, in the egg white. We have to look through the egg white through the yolk to actually make an image of this black hole. And there’s a lot of gas and dust … that scatter and blur the ring-like feature and that has to be accounted for in the imaging process and in the calibration process. That took a lot of effort to solve.”
In their five-year effort, the team of researchers developed complex tools to overcome the problems of imaging Sgr A*, using supercomputers to combine and analyse their data, all while compiling an unprecedented library of simulated black holes to compare with the observations. The image of the Sgr A* black hole is an average of the different images the team extracted.
Among the more than 300-strong research team, Deane and Wits postdoctoral fellow Dr Iniyan Natarajan were the only scientists based on African soil. Their 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.
Deane said the Wits Centre for Astrophysics is proud to be part of the global EHTC, which had unveiled the historic result. “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.”