On May 12, 2022, the Event Horizon Telescope (EHT) Collaboration released the first-ever image of the supermassive black hole at the center of our own Milky Way galaxy, attracting global media attention. Black holes are the most compact and energetic known objects in the universe and are by their very nature extremely challenging to study.
Scientists release first image of giant black hole at Milky Way's galactic center
Pictured above: The first-ever image of Sagittarius A* (or Sgr A* for short), the supermassive black hole at the center of the Milky Way galaxy, reveals a glowing gaseous ring surrounding a black ‘shadow,’ which is the black hole. Captured by the Event Horizon Telescope collaboration, it is the first direct visual evidence of the presence of this black hole. The image is an average of the different images the EHT Collaboration extracted from its April 2017 observations of the galactic center. Image courtesy of EHT Collaboration.
Siv Schwink
for Illinois Physics
On May 12, 2022, the Event Horizon Telescope (EHT) Collaboration released the first-ever image of the supermassive black hole at the center of our own Milky Way galaxy, attracting global media attention. Black holes are the most compact and energetic known objects in the universe and are by their very nature extremely challenging to study. Beyond its capturing the imagination of the public, what does this latest black hole image tell astronomers and astrophysicists about the nature and history of the universe?
The image of Sagittarius A* (Sgr A*, pronounced “sadge-A-star”) was reconstructed from observations taken in April 2017, using an array of eight existing radio telescopes positioned around the globe—all pointed at the same target, at the same time. In this way, the EHT array forms a virtual Earth-sized telescope, capable of delivering the unprecedented resolution and sensitivity needed to investigate the massive compact object at the galactic center, 27,000 light-years away from Earth. The image is the first-ever direct visual evidence that this object is indeed a supermassive black hole.
This achievement reflects the ingenuity of the more than 300 researchers from 80 institutes around the world who together comprise the EHT Collaboration. Processing the huge collection of Sgr A* data and developing the algorithms to reconstruct the image took EHT scientists five years, which was longer than anticipated—the accretion flows surrounding Sgr A*’s event horizon are turbulent, rapidly changing, and exceedingly complex.
Illinois Physics and Astronomy Professor Charles Gammie co-leads the EHT theory working group. He and his research group in Urbana contributed a vast library of numerical models to this effort, which were compared with EHT’s Sgr A* data, to elucidate the astrophysics observed.
“My group and I have spent more than a decade developing key techniques for building models of Sgr A star,” notes Gammie. “The models do a terrific job of explaining nearly all the data. But we uncovered a mystery—Sgr A star is much quieter than we expected.”
The state-of-the-art simulations developed by the Gammie group in Urbana use general relativistic magnetohydrodynamics (GRMHD) to solve for the flow of hot gas around the black hole. These numerical solutions are then fed to general relativistic ray-tracing algorithms to generate predicted images of the black hole.
“Our team at Illinois works to explain what the image means,” says Gammie. “Black holes can spin like a flywheel. Does the image tell us anything about the black hole spin and the direction it’s rotating? How strong are the magnetic fields around the black hole?”
By comparing these predicted images with the horizon-scale resolution images taken by the EHT, scientists are also finally able to ask, does Einstein’s general theory of relativity correctly predict the spacetime near a supermassive black hole?
“To answer these questions,” Gammie continues, “we build computer models that track the flow of gas into the black hole and then make synthetic observations. Many of the beautiful animations you saw in the press release are based on our computer models. We then compare the models to the data using sophisticated statistical techniques.”
Gammie says EHT scientists gleaned three key scientific takeaways by comparing the Sgr A* image with the numerical models.
HOW THE SAGITTARIUS A* IMAGE WAS MADE
The Event Horizon Telescope (EHT) Collaboration created a single image (top frame) of the supermassive black hole at the center of our galaxy, called Sagittarius A* (or Sgr A* for short), by combining images extracted from EHT observations.
The main image was produced by averaging together thousands of images created using different computational methods—all of which accurately fit the EHT data. This averaged image retains features more commonly seen in the varied images, and suppresses features that appear infrequently.
The images can also be clustered into four groups based on similar features. An averaged, representative image for each of the four clusters is shown in the bottom row. Three of the clusters show a ring structure having differently distributed brightness around the ring. The fourth cluster contains images that also fit the data but do not appear ring-like.
The bar graphs show the relative number of images belonging to each cluster. Thousands of images fell into each of the first three clusters, while the fourth and smallest cluster contains only hundreds of images. The heights of the bars indicate the relative “weights,” or contributions, of each cluster to the averaged image.
Image courtesy of EHT Collaboration.
“We learned a lot from this result,” notes Gammie. “First, the image is a ring. It’s consistent with what we expected, based on earlier Nobel Prize–winning measurements by Andrea Ghez of UCLA and Reinhard Genzel of the Max Planck Institute in Germany. They followed the orbits of stars around the center of the galaxy over decades. That allowed a very precise measurement of the black hole mass and distance, which in turn allowed us to predict how big the ring should be.
“Second, the image is consistent with Einstein’s theory of gravity. It could have been much bigger or smaller, and it’s not—it’s right on the button.
“Third, we interpret the image by comparing it to simulations, and although we’re less confident about this, the models that match the best are nearly face-on (in other words, we’re looking nearly at the pole of the black hole), have a spinning black hole, and have strong magnetic fields.”
In 2019, the EHT collaboration released the first-ever image of a black hole, and it is strikingly similar to this latest image: a glowing gaseous ring at the event horizon surrounds a dark “shadow,” which is the black hole. Gammie says the much larger M87* black hole—tens of millions of light years away at the center of the more distant Messier 87 galaxy—was actually easier to capture than Sgr A*.
“Our black hole is a thousand times smaller than the black hole in Messier 87, but also a thousand times closer, so it appears about the same size from our vantage point on Earth,” explains Gammie.
Black hole mass scales with size, unlike the mass of other celestial objects such as planets and stars.
“Because the black hole is a thousand times smaller,” Gammie continues, “the pattern of glowing gas around the black hole changes a thousand times more quickly in our galaxy than in Messier 87. This makes it really hard to get a clear picture. Where Messier 87 posed for us like an old great Dane, our Sgr A* is like a puppy chasing its tail—it won’t sit still!”
The researchers developed sophisticated new analytical tools to account for the gas movement around Sgr A*. The latest image is an average of the thousands of different images the EHT collaboration captured of the galactic center during its April 2017 observations. The picture was made using a technique called VLBI, which stands for “very long baseline interferometry”: data from pairs of telescopes were compared and used to generate images.
It was a five-year process—and all told, a huge step forward in black hole physics.
“This result is an important building block in our understanding of black holes and the role they play in our galaxy and in our universe,” Gammie sums up. “More broadly, it’s just an exciting result. Kids see it, and it raises their interest in astronomy and in science more broadly.”
Notes from the Gammie EHT research team in Urbana
“Our simulations model the motion of a fluid. In our case, the fluid is an ionized plasma with a magnetic field that experiences the effects of the strong gravity near a black hole. We generated on the order of thousands of models, simulating a huge range of parameters, and we found that only a couple really match the picture very closely. This is exciting—we have simulations that can accurately describe the region very close to the black hole. And even more exciting is that none of the simulations work perfectly, which is probably a sign that there are ingredients missing from our models. We can continue to look at the aspects that don’t match the observations and work to figure out, what’s the missing piece in our simulations?”
“It’s been a joy to study these astronomical mysteries with so many scientists from around the globe. And it’s wonderful to see how many people are learning about black holes. Recently, we spoke at a local elementary school, and every one of the kids knew about the first photo of a black hole. Now that we’ve revealed the first photo of a black hole in our own galaxy, we’ve been getting even more questions. It’s incredible to be working on something that might inspire the next generation of scientists!”
“Working toward the Sagittarius A* results as a member of the Event Horizon Telescope Collaboration was an exhilarating and humbling journey. As theoreticians we generated hundreds of models and examined over a million synthetic images. Analyzing these data and coordinating with collaborators across the globe in the midst of a global pandemic has been a feat in itself. Personally, I have learned a lot from the discussions with fellow members, and it has been an immensely gratifying experience.”
“Physics and astronomy were always interesting subjects in school, but to be a part of such a significant discovery is very surreal to me. It’s both awe-inspiring and humbling to see how so many brilliant collaborators around the world (and here at UIUC) are able to work together to produce both sound and profound results. I hope there are others around the world who are inspired by this work and will pursue their dreams in astrophysics.”
“It’s been an incredible experience working with the Event Horizon Telescope. Beyond just the scientific challenges, we also needed to work with the struggles of coordinating and organizing a large, global collaboration of scientists in such a monumental undertaking, especially in times of COVID. It’s been very exciting to collaborate with teams across the world on small projects and to watch our efforts slowly but surely coalesce into the results that we’ve put out—not only the beautiful image of Sgr A* but also the wealth of scientific data that came with these observations.”
“I think the most exciting thing about this result has been the sheer scale! The analysis effort has involved hundreds of thousands of simulated images from dozens of large-scale computer simulations, covering a huge breadth of possibilities for what might be producing the emission that the EHT sees. Working on generating and analyzing this much data has been a fun challenge. The scale of collaboration and community effort that has gone into these results is also immense. We’ve worked with teams all over the world contributing models and ideas together, filling out a whole mosaic of possibilities to evaluate against the observational results.”
The Gammie group's work on the EHT Collaboration experiment is funded by the National Science Foundation under Grant Nos. NSF AST 17-16327, NSF OISE 17-43747, and NSF AST 20-34306. The results presented are those of the researchers and not necessarily those of the funding agency.