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Astronomers have unveiled the first image of the supermassive black hole at the centre of our own Milky Way galaxy. Among the team are several UvA scientists. 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 was produced by a global research team called the Event Horizon Telescope (EHT) Collaboration, using observations from a worldwide network of radio telescopes.

This is the first image of Sagittarius A* (or Sgr A* for short), the supermassive black hole at the centre of our galaxy. It’s the first direct visual evidence of the presence of this black hole. It was captured by the Event Horizon Telescope (EHT), an array which linked together eight existing radio observatories across the planet to form a single “Earth-sized” virtual telescope. The telescope is named after the “event horizon”, the boundary of the black hole beyond which no light can escape.

The image was made through international collaboration within the Event Horizon Telescope and has been presented today by EHT-director Huib Jan van Langevelde (JIVE/Universiteit Leiden) on a press conference in the headquarters of the European Southern Observatory (ESO) in Germany. In Amsterdam, the foyer of the beta faculty of the UvA hosted an event to highlight the results.

The team used observations from a global network of radio telescopes. Dutch astronomers and technicians of the University of Amsterdam, the Radboud University, the University of Leiden, University of Groningen, JIVE and ASTRON are among the people involved in the  EHT collaboration. UvA scientists participating in the project are Sera Markoff, Oliver Porth, Koushik Chatterjee, Gibwa Musoke, Doosoo Yoon, Nathalie Degenaar and Rudy Wijnands.

‘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, 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.’

The EHT team's results are being published today in a special issue of The Astrophysical Journal Letters.

First proof of Sagitarius A*

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 telltale 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.

Donut on the moon

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 a donut 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 [1]. The EHT observed Sgr A* on multiple nights, collecting data for many hours in a row.

Much alike

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* [2]. ‘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,’ says Sera Markoff, Co-Chair of the EHT Science Council and a professor of theoretical astrophysics at the University of Amsterdam, the Netherlands. ’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.’

This achievement was considerably more difficult than for M87*, even though Sgr A* is much closer to us. EHT scientist Chi-kwan (‘CK’) Chan, from Steward Observatory and Department of Astronomy and the Data Science Institute of the University of Arizona, US, explains: ‘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 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.

20 year old prediction

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.

In 2000, Heino Falcke, member of the EHT board and professor of astroparticle physics and astronomy at the Radboud University, coined the term ‘black hole shadow’ for the effect of light distortion around a black hole, and predicted that it should be possible to image a black hole with radio telescopes. He and Sera Markoff then also created the first formulas describing the radiation from this black hole. ‘It looks like our predictions of 20 years ago are now being confirmed’, says Markoff.

‘To extract information on the underlying physics from the image, we put the equations describing the behaviour of light and matter into the computer. Our predictions come remarkably close to the EHT image’, says Oliver Porth, EHT scientist and Assistant Professor for theoretical simulations at University of Amsterdam. ‘But what is even more exciting is that once we combined the EHT observation with our knowledge about the Galactic centre gathered over decades, it becomes extremely difficult to get everything right. This is fascinating because we can now learn something new about the behaviour of matter near black holes.’

Two black holes observed

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, Taipei. ‘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.’

Africa Millimetre Telescope

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. The Radboud University, together with the University of Namibia, is developing the Africa Millimetre Telescope (AMT) on the Gamsberg in Namibia. The UvA group is also involved in the scientific part of the project and hopes to contribute by requesting grants. The AMT telescope, expected to be ready in 2026, is a crucial addition to the network for making dynamic recordings of black holes.

Contacts

Geoffrey Bower, EHT Project Scientist, Institute of Astronomy and Astrophysics, Academic Sinica, Taipei, Tel: +1-808-961-2945, Email: gbower@asiaa.sinica.edu.tw

Huib Jan van Langevelde, EHT Project Director, JIVE and University of Leiden, The Netherlands, Mobile: +31-62120 1419, Email: langevelde@jive.eu

Sera Markoff, Vice-president EHT science council, Universiteit van Amsterdam, 06-28547250, S.B.Markoff@uva.nl

Heino Falcke, EHT-board, Radboud Universiteit, +49 1512 3040365, h.falcke@astro.ru.nl

Communications, NOVA/Marieke Baan, h.m.baan@uva.nl, 0614322627