First images of black hole at the centre of our galaxy
Using a global network of radio telescopes, scientists have made the first images of two supermassive black holes. The first, in the centre of the galaxy Messier 87, 55 million light-years away, was released in 2019. The second was a stunning image of the black hole at the centre of our own Milky Way galaxy, called Sagittarius A* (Sgr A*), released in 2022.
These images highlight the glowing material accreting around the event horizon of each black hole, where gas is heated until it becomes plasma and emits energy in radio wave frequencies. But while Einstein’s general relativity still seems to hold true even in these extreme conditions, the plasma seems to be calmer than expected.
Dr Po Kin Leung, Lecturer, and Mr Tin Lok Chan, an undergraduate student, both from The Chinese University of Hong Kong (CUHK)’s Department of Physics, were part of the team that helped create the images. They worked with Professor Chi-Kwan Chan, Associate Research Professor at the University of Arizona who was an exchange scholar at CUHK, to model the plasma in the event horizon.
Leung says: “So far the results are both exciting and boring. It’s the first time in history we’ve been able to resolve an image of such an extreme environment, but Einstein was again correct about how general relativity works. However, our model overpredicted the variability in how the plasma moves and brightens.”
Black holes are extremely dense, causing such strong gravity that even light cannot escape the event horizon – the point of no return for matter falling in. Despite indirect evidence pointing to a supermassive black hole at the centre of our galaxy, astronomers have never been able to observe it directly.
One of the difficulties in observing Sgr A* is that it requires a telescope with very high resolution. Since Sgr A* is about 26,000 light-years away from Earth, trying to look at it is like trying to see an apple on the Moon from Earth.
The Event Horizon Telescope (EHT) is a collaboration of more than 300 researchers and engineers around the world, using eight radio telescopes. These work together as a massive interferometer – a technology that compares the differences in how and when radio waves arrive at each separate telescope to resolve an image.
The radio waves are produced when the gas surrounding the event horizon is bent by the black hole’s powerful gravity, which heats it to the billions of degrees. However, there are many ways the data can be interpreted, so part of Leung, Chan and Chan’s work was to model the plasma using supercomputers, and to match the simulations to the observations.
This work showed that the simulations were very successful in predicting the image – except when it came to predicting the variability of the plasma. The team are now working with the observation datasets to tweak the theory behind the simulations, leading to a better understanding of how the plasma varies. It may be that one of the factors needs updating, such as the viscosity of the plasma, or how it emits radio waves in relation to its temperature.
This mystery is only the beginning of what researchers hope to discover using the EHT. The next step for the two black holes already imaged is to make image collection easier, so enough can be captured over time to create movies of how they change.
To image other black holes, however, telescopes further apart are needed to get enough resolution – meaning we need to get radio telescopes in space. This is a longer-term goal but would help image different kinds of black holes, and a proposal is already in the works with NASA.
For example, there are black holes thought to spin so fast that the event horizon disappears. If these objects were seen by the future EHT, they could reveal new insights in the singularity at the centre – the region where general relativity is believed to break down.
The technology even has more down-to-Earth applications. Chi-Kwan Chan says: “From the fundamental science 100 years ago to the new discoveries, the quest to investigate black holes has acted like a start-up: a lot of smart people have got together to create some exceptional technology. And that technology has been applied beyond astronomy; the method for reconstructing images can be used to improve medical imaging, meaning patients are exposed to damaging X-rays for a shorter time.”
But it’s the allure of astronomy that keeps the team going. Leung says: “It’s fascinating how humans can use our rational minds to understand the Universe – how we can get it to reveal itself, even though we are light-years away from the nearest object.”