The first image of the supermassive black hole at the centre of our own Milky Way galaxy has been produced by a global team involving UCL researcher Dr Ziri Younsi.
The 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, produced by the Event Horizon Telescope (EHT) Collaboration, uses observations from a worldwide network of radio telescopes, and 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 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, and today’s image provides the first direct visual evidence of it.
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.
Dr Ziri Younsi (UCL Mullard Space Science Laboratory), EHT Collaboration member and co-author on the research papers describing the new findings, said: "Our results are the strongest evidence to date that a black hole resides at the centre of our galaxy. This black hole is the glue that holds the galaxy together. It is key to our understanding of how the Milky Way formed and will evolve in the future.
"Producing this image is the result of a monumental effort by hundreds of scientists over five years. It was especially challenging because of the haze of stars, dust, and gas in between Earth and the Galactic Centre, as well as the fact that the pattern of light from Sgr A* changed quickly, over the course of minutes. But now we have comprehensive findings, and this work opens a new chapter in our understanding of black holes."
The effort was made possible through the ingenuity of more than 300 researchers from 80 institutes around the world that together make up the EHT Collaboration. In addition to developing complex tools to overcome the challenges of imaging Sgr A*, 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.
Dr Younsi, a UKRI Stephen Hawking Fellow at UCL who has recently been appointed to the EHT Science Council, helped run many of these simulations, and is the author of one of only two codes used to generate the entire library of nearly 2 million simulated images of Sgr A*.
EHT Project Scientist Dr Geoffrey Bower, from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei and University of Hawai’i at Manoa, US, said: "We were stunned by how well the size of the ring agreed with predictions from Einstein’sTheory of General Relativity. These unprecedented observations have greatly improved our understanding of what happens at the very centre of our galaxy."
The EHT team’s results are being published today in a special issue of The Astrophysical Journal Letters.
Because the black hole is about 27,000 light-years away from Earth, it appears to us to have about the same size in the sky as an orange 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, collecting data for many hours in a row, similar to using a long exposure time on a camera.
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.
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*.
Professor Sera Markoff, Co-Chair of the EHT Science Council based at the University of Amsterdam, the Netherlands, said: " 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. 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 surrounding material."
EHT scientist Dr Keiichi Asada from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei, said: "Now we can study the differences between these two supermassive black holes [and] can go much further in testing gravity in these extreme environments than ever before."
This achievement was considerably more difficult than for M87*, even though Sgr A* is much closer to us. EHT scientist Dr Chi-kwan (’CK’) Chan, from Steward Observatory, Department of Astronomy of the University of Arizona, US, said: "The gas in the vicinity of both black holes orbits at nearly light speed. But where gas takes days to weeks to orbit the larger M87*, for the much smaller Sgr A* an orbit completes in mere minutes. This means the brightness and pattern of the gas around Sgr A* changes rapidly as the EHT observes it."
The researchers had to develop sophisticated new tools that accounted for the gas movement around Sgr A*. 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 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.
Progress on the EHT continues: a major observation campaign in March 2022 included more telescopes than ever before. 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.