Pictured: the black hole at the heart of our galaxy

Astronomers last night unveiled the first image of the supermassive black hole at the centre of the Milky Way galaxy, providing overwhelming evidence that the object in question is indeed a black hole and yielding valuable clues about the workings of such giants, which are thought to reside at the centre of most galaxies.

The image was produced by a global research team called the Event Horizon Telescope (EHT) Collaboration, comprising more than 300 researchers from 80 institutes, using observations from a worldwide network of radio telescopes. The team’s results have been published in a special issue of The Astrophysical Journal Letters.

Scientists had previously 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*, pronounced ‘sadge-ay-star’) — is a black hole. The new image provides the first direct visual evidence of this.

Although we cannot 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,” said EHT project scientist Geoffrey Bower from the Institute of Astronomy and Astrophysics, Academia Sinica. “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.”

Because the black hole is about 27,000 light-years away from Earth, it appears to us on Earth to have about the same size in the sky as a doughnut on the Moon. 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 in 2017, collecting data for many hours in a row, similar to using a long exposure time on a camera.

The achievement follows the collaboration’s 2019 release of 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 1000 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,” said Sera Markoff, Co-Chair of the EHT Science Council and a professor of theoretical astrophysics at the University of Amsterdam. “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.”

The latest achievement was considerably more difficult than for M87*, even though Sgr A* is much closer to us. EHT scientist Chi-kwan (‘CK’) Chan, from the Department of Astronomy and Steward Observatory at the University of Arizona, explained: “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* were 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.”

So while M87* was an easier, steadier target, with nearly all images looking the same, that was not the case for Sgr A*. The image of Sgr A* is thus an average of the different images the team extracted, which required the development of complex new tools to account for the gas movement around the black hole, the use of supercomputers to combine and analyse the data, and the compilation of an unprecedented library of simulated black holes to compare with the observations.

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,” said EHT scientist Keiichi Asada from the Institute of Astronomy and Astrophysics, Academia Sinica. “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.”

Image shows Sgr A*, the supermassive black hole at the centre of our galaxy. It’s the first direct visual evidence of the presence of this black hole. Image credit: EHT Collaboration.

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