During the press conference organized today by the National US Science Foundation, along with the Event Horizon Telescope Collaboration, astronomers have published the first image of a supermassive black hole in the center of our Milky Way galaxy.
This result provides convincing evidence that the object is indeed a black hole, and provides valuable insight into the work of such giants, which are believed to be at the center of most galaxies. The image was created by a global research team called the Event Horizon Telescope, or EHT Collaboration, using observations from a worldwide network of radio telescopes.
The image is a long-awaited view of a massive object at the heart of our galaxy. Earlier, scientists have seen stars orbiting something invisible, compact and very massive in the center of the Milky Way. This suggests that this object – known as Sag A (pronounced “sadge-ay-star”) is a black hole, and today’s image is the first direct visual evidence of this.
Although we cannot see the black hole itself because it is completely dark, the gas glowing around it shows a characteristic feature: a dark central area (so-called “shadow”), surrounded by a bright annular structure. The new look captures light bent by the powerful gravity of a black hole that 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, 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 results are published today in a special issue of The Astrophysical Journal Letters.
Because a black hole is about 27,000 light-years from Earth, it seems to us that it is about the same size in the sky as a donut on the moon. To image it, the team created the powerful Event Horizon Telescope, which brought together eight existing radio observatories around the world to create a single Earth-sized virtual telescope. EHT observed Sgr A for several nights, collecting data for many consecutive hours, similar to the use of long-term exposure on camera.
The breakthrough follows the release in 2019 of the joint EHT first image of a black hole called M87 in the center of the more distant galaxy Messier 87. The two black holes look remarkably similar, although the black hole in our galaxy is more than a thousand 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,” 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 much more difficult than for the M87, although the Sgr A is much closer to us. EHT scientist Chi-Kwan (“CK”) Chan of the Steward Observatory and the Department of Astronomy and the Institute of Data Science at the University of Arizona, USA, 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.”
Researchers had to develop new sophisticated tools to explain the movement of gas around Sgr A. Although the M87 was a lighter and more stable target, almost all images looked the same, for Sgr A this was not the case. The image of the black hole Sgr A is the average of the various images obtained by the team, and finally shows for the first time a giant hiding in the center of our galaxy.
This effort has been made possible by the ingenuity of more than 300 researchers from 80 institutes around the world who make up the EHT Collaboration. In addition to developing sophisticated tools to overcome the challenges of creating Sgr A images, the team worked diligently for five years using supercomputers to combine and analyze their data, compiling an unprecedented library of simulated black holes for comparison with observations.