On Thursday last week, NASA’s Event Horizon Telescope (EHT) captured the first ever image of the supermassive black hole at the centre of our galaxy. The imaging of the supermassive black hole, named Sagittarius A* (Sgr A*), provides strong evidence to support the 60-year-old theory that a supermassive black hole lurks at the centre of the Milky Way.
The release of the picture is the latest in a series of developments in our knowledge of these elusive and mysterious objects – and come three years after the imaging of the supermassive black hole in the centre of the Messier 87 galaxy (M87*).
Sagittarius A* lies in the centre of the Milky Way 26,000 lightyears away from Earth and can be seen in the night sky as part of the Sagittarius constellation. Despite being four million times more massive than our Sun, Sagittarius A* is still about 1000 times smaller than the M87 supermassive black hole.
More than 300 astronomers, and hundreds of engineers and support staff from 60 institutions across 20 countries and regions, processed data from a 2017 observation of Sagittarius A*.
Referred to as an “Earth-sized telescope,” the EHT links together 11 telescopes around the world, effectively creating one telescope with a mirror the size of the Earth. The EHT detects radio frequencies to create the image of Sagittarius A*. The bright orange ring in the image is the matter swirling around the black hole, and the dark shadow in the middle is the black hole itself.
“While the Earth is rotating, all telescopes observe the same astronomical object for several hours,” explains Thomas P Krichbaum, of Germany’s Max Planck Institute, at a press conference to announce the findings. “At each telescope, the data are recorded on hard disks and are accurately time tagged by precise atomic clocks. After observations, the data are shipped to processing centres where they are combined in supercomputers.
“After a number of quite complex data analysis steps, this results in the high-resolution image of the radio source.”
Though captured at the same time, the image of Sagittarius A* took longer to complete than the image of M87*. This is because Sagittarius A* is constantly changing with matter orbiting it in a matter of minutes. Comparatively, matter orbits M87* over the course of days. So, imaging Sagittarius A* clearly is not easy work.
“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*,” says Chi-kwan Chan, an EHT scientist based at the University of Arizona, US. “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.”
Xavier Barcons, director general of the European Southern Observatory, spoke at the press conference: “This extraordinary result would not have been possible to achieve by one single facility or even the national astronomical community of a single country. It took eight radio observatories around the world, and that network has already expanded to 11 today, many built, funded, operated and supported through international organisations across many countries around the world.”
Barcons added that the discovery “shows what we can achieve when we cooperate, when we work together. This is very important to remember in the times that we are living in, where the world is not running in that direction unfortunately”.
Super dense objects whose gravitational pull not even light can escape, black holes are extreme examples of Einstein’s Theory of General Relativity.
“We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity,” says EHT 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.”
Other astronomers and black hole experts have reacted to the image of Sagittarius A*.
Science director at Australia’s Curtin University node of the International Centre for Radio Astronomy Research, Professor James Miller-Jones, praised the “quite remarkable feat in imaging the supermassive black hole at the centre of our Milky Way galaxy”.
“Excitingly, the near-perfect agreement of the image with the theoretical predictions shows that Einstein’s General Relativity has passed yet another stringent test, and the similarity of the image with that of M87 provides further confidence in our understanding of how a black hole looks and behaves,” he says. “However, this is only a first glimpse into the black hole at the centre of our Milky Way; ongoing telescope upgrades should provide sharper images, and the next-generation facility being planned is aiming to provide real-time movies of black holes, allowing us to study the turbulent environment around these exotic objects.”
The imaging of Sagittarius A* is another exciting step forward as scientists seek to understand these mysterious and extreme objects.
Last week, Cosmos reported that NASA had released the sounds of black holes, including the music produced by M87*. Scientists involved in the sonification projects spoke with Cosmos.
Using NASA’s Chandra X-ray Observatory, astronomers reproduced the sound produced by the supermassive black hole at the centre of the Perseus galaxy cluster. “Clusters of galaxies contain huge amounts of hot gas – with temperatures of tens of millions of degrees – between the galaxies, which produces X-rays detected by Chandra,” explains Peter Edmonds, senior astrophysicist with the Harvard and Smithsonian-based Center for Astrophysics in the US.
“In the case of the Perseus galaxy cluster, astronomers used the Chandra X-ray data to detect a pattern of ripples in the hot gas, which they interpreted as sound waves generated by outbursts from the supermassive black hole at the centre of the cluster. The regular spacing corresponds to an extraordinarily deep note.”
The sounds from the Perseus black hole “tell us that black holes can produce regular, powerful eruptions of material, and they also tell us how long this activity can last,” says Edmonds.
Kim Arcand, also from the Center for Astrophysics says: “Engaging more than visual senses with astrophysical data may be beneficial for both research and communication.
“We are interested in exploring this potential and working on new techniques to squeeze all of the sciencey goodness out of the data available to us of our rather fascinating and certainly mysterious universe.
“It’s a whole other way to consider data of our universe, and I get particularly excited about the potential to breathe some new life into archival data,” Arcand adds. “We are starting to research some techniques to help us potentially compare the geometries of astrophysical objects through sonification, whether radial slices moving through a cluster, or stepping along an edge-on galaxy, etc.”
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