Astronomers Reveal a Black Hole at the Heart of our Galaxy

A black hole at the center of our galaxy.

The first image of the black hole located at the center of our own Milky Way galaxy, called Sagittarius A*, was unveiled Thursday by the international Event Horizon Telescope Collaboration. The discovery would not have been possible without the groundwork and heavy lifting provided by UArizona scientists and facilities. (Image:EHT Collaboration)

Astronomers have unveiled the first image of the supermassive black hole at the center of our own Milky Way galaxy. 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 center of most galaxies. The image was produced by a global research team called the Event Horizon Telescope Collaboration, using observations from a worldwide network of radio telescopes. Researchers at the University of Arizona played a leading role in the effort, providing two of the eight telescopes used to make the observations and performing data analysis that resulted in the image unveiled Thursday.

The image is a long-anticipated look at the massive object that sits at the very center of our galaxy. Scientists had previously seen stars orbiting around something invisible, compact, and very massive at the center of the Milky Way. This strongly suggested that the object — known as Sagittarius A*,  or Sgr A* (pronounced “sadge-ay-star”) — is a black hole, and the new image provides the first direct visual evidence of it.

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This image of Sagittarius A* is the first direct visual evidence of the presence of a supermassive black hole at the center of our galaxy.
This image of Sagittarius A* is the first direct visual evidence of the presence of a supermassive black hole at the center of our galaxy. It was captured by the Event Horizon Telescope, an array of radio telescopes named after the ‘event horizon,’ the boundary of the black hole beyond which no light can escape. (Image: via EHT Collaboration)

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 ringlike structure. The new view captures light bent by the powerful gravity of the black hole, which is 4 million times more massive than the sun. EHT project scientist Geoffrey Bower from the Institute of Astronomy and Astrophysics at Academia Sinica in Taipei, Taiwan, said:

“We were stunned by how well the size of the ring agreed with predictions from Einstein’s theory of general relativity. These unprecedented observations have greatly improved our understanding of what happens at the very center of our galaxy and offer new insights on how these giant black holes interact with their surroundings.”

This animation shows the locations of some of the telescopes making up the EHT, as well as a representation of the long baselines between the telescopes. (Credit: ESO/L. Calçada)

The EHT team’s results are published in a special issue of The Astrophysical Journal Letters. 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. The EHT observed Sgr A* on multiple nights, collecting data for many hours in a row, similar to using a long exposure time on a camera.

The Submillimeter Telescope on Mount Graham in Arizona and the South Pole Telescope in Antarctica — both under the leadership of Dan Marrone, a professor of astronomy and an astronomer at the UArizona College of Science‘s Steward Observatory — proved that an Earth-sized telescope would work. Under the leadership of Chi-kwan “C.K.” Chan, an associate research professor at Steward, the EHT team at UArizona developed powerful computational models capable of predicting what sources like Sgr A*, which change over time, would look like in the sky.

The main image of Sgr A* was produced by averaging together thousands of images created using different computational methods. Representations from four image clusters are shown in the bottom row. Thousands of images fell into each of the first three clusters, which show a ring structure. The fourth and smallest cluster contained only hundreds of images that did not appear ring-like. (Image: via EHT Collaboration)

Black holes in the center of two galaxies

The Sgr A* image breakthrough follows the EHT Collaboration’s 2019 release of the first image of a black hole, called M87*, at the center of the more distant Messier 87 galaxy. The two black holes look remarkably similar, even though our galaxy’s black hole is more than 1,000 times smaller and less massive than M87*. Sera Markoff, co-chair of the EHT Science Council and a professor of theoretical astrophysics at the University of Amsterdam in the Netherlands, said:

“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. 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 animation shows theoretical models of the black hole at the center of our galaxy. The Event Horizon Telescope (EHT) Theory Working Group built hundreds of models and then compared them one by one to measurements made by the EHT and other telescopes. The animation shows a build-up as more and more models are added to the library, and then a cut-down as they are eliminated by comparison to the data. The final image shows a high-resolution image of our best-bet model, which features a spinning black hole surrounded by a strongly magnetized plasma, seen nearly face-on. (Image: via EHT Theory Working Group)

This achievement was considerably more difficult than for M87*, even though Sgr A* is much closer to us, explained Chan, adding:

“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 South Pole Telescope is located at the National Science Foundation's Amundsen-Scott South Pole Station, Antarctica, the most extreme location of the eight telescopes in the Event Horizon Telescope array during the 2017 observation run.
The South Pole Telescope is located at the National Science Foundation’s Amundsen-Scott South Pole Station, Antarctica, the most extreme location of the eight telescopes in the Event Horizon Telescope array during the 2017 observation run. It is one of two in the array integrated by the University of Arizona. (Image: Junhan Kim via University of Arizona)

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 center of our galaxy for the first time. 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 analyze data, all while compiling an unprecedented library of simulated black holes to compare with the observations. Elizabeth “Betsy” Cantwell, UArizona senior vice president for research and innovation, said:

“This is an exciting day for astrophysics, for the University of Arizona and for all of those fascinated and curious about our galaxy. This effort highlights not just the immense complexity of and commitment toward such a discovery, but also the ingenuity of our University of Arizona scientists and the more than 300 other researchers from dozens of institutions around the world that together make up the EHT Collaboration.”

What does it take to capture an image of the black hole at the center of our galaxy? This video explains how the Event Horizon Telescope (EHT) works, and how astronomers managed to create one massive Earth-sized telescope big enough to ‘see’ at the edge of black holes. (Image: via ESO)

A founding partner of the EHT Collaboration, UArizona has provided significant resources and leadership in terms of the EHT’s footprint, intellectual contributions, and publications. In the early days of the project, the National Science Foundation funded the EHT with $28 million, a large portion of which was awarded to UArizona. Dimitrios Psaltis, a UArizona professor of astronomy and physics and principal investigator of the international Black Hole PIRE Project, served as the founding project scientist. Remo Tilanus, a research professor and astronomer at Steward Observatory, serves as operations manager of the Event Horizon Telescope.

The Submillimeter Telescope, on Mount Graham in Arizona, is one of eight telescopes in the Event Horizon Telescope array that made it possible to capture the black hole image.
The Submillimeter Telescope, on Mount Graham in Arizona, is one of eight telescopes in the Event Horizon Telescope array that made it possible to capture the black hole image. It’s one of two in the array integrated by the University of Arizona. (Image: Paul Schulz via University of Arizona )

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. EHT scientist Keiichi Asada from the Institute of Astronomy and Astrophysics at Academia Sinica in Taipei, Taiwan, said:

“Now we can study the differences between these two supermassive black holes to gain valuable new clues about how this important process works. 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.”

While they may look similar, the two black holes imaged by the Event Horizon Telescope (EHT) Collaboration are actually very different. Sagittarius A* (Sgr A*), the black hole at the center of the Milky Way, is over a thousand times smaller than the black hole at the center of the galaxy M87. The reason they appear the same size is because of their relative distances from Earth. This animation demonstrates their massive differences in size by showing what the M87 black hole would look like if it were closer to Earth. The final frame shows the sizes of the two black holes if they were both at the same distance from Earth as Sgr A*. (Image: ESO / M. Kornmesser via EHT Collaboration)

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. Feryal Özel, professor of astronomy and physics at UArizona’s Steward Observatory, has been a member of the EHT Science Council since its inception and has led the modeling and analysis group, said:

“We’ve gotten into everything from very basic theoretical analytic models, all the way to extremely complex, large computational efforts. To do this kind of work, you need new hardware, new types of algorithms and new types of models.”

UArizona researchers Dimitrios Psaltis, Feryal Özel, Chi-Kwan Chan, and Dan Marrone.
From left: UArizona researchers Dimitrios Psaltis, Feryal Özel, Chi-Kwan Chan, and Dan Marrone are all members of the Event Horizon Telescope Collaboration. (Image: Chris Richards via University of Arizona)

EHT is a very organic collaboration, according to Özel, with partners bringing their own expertise, resources, and telescopes in the face of limited funding. UArizona has led many of these efforts, she added. As early as the late 1990s, Özel and Psaltis —  concurrently with another team that included Fulvio Melia, a professor in the Department of Physics — undertook the first studies exploring the feasibility of taking a picture of Sgr A*. In November 2012, Psaltis, Marrone, and Özel hosted the inaugural meeting of the Event Horizon Telescope in Tucson, which established the worldwide collaboration, Psaltis said:

“Many of the early ideas came out of here, the computational tools and infrastructure for the simulation library came out of here, the proof of principle for testing Einstein’s theory came out of here. Everything, from running the telescopes to analyzing the data, is a collective effort. Each one of us has their hands in basically everything, and we made it work.”

Provided by Daniel Stolte, University of Arizona [Note: Materials may be edited for content and length.]

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