Astronomers Capture First Image of Black Hole at the Center of the Milky Way

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The black hole Sagittarius A*. Credit: EHT

Three years after sharing the first ever picture of a black hole, researchers with the Event Horizon Telescope (EHT) collaboration have revealed a groundbreaking discovery that hits much closer to home: the first image of the supermassive black hole at the heart of our Milky Way galaxy.

The long-awaited image is confirmation that the massive object at the center of the Milky Way—known as Sagittarius A*—is indeed a black hole. For decades, scientists and astronomers have seen stars orbiting around something invisible, compact and very massive. While these signs strongly suggested the object was a black hole, astronomers were unable to confirm that fact—until now.

“For me personally, I met [Sagittarius A*] 20 years ago and have loved it and tried to understand it since. But until now, we didn’t have the direct picture confirming that Sagittarius A* was a black hole. Today, the EHT is delighted to share the first direct image of the gentle giant at the center of our galaxy,” Feryal Ozel, astronomy professor at University of Arizona, said during Thursday morning’s press conference announcing the discovery.

Since it is impossible to see a completely dark black hole itself, the captured image shows the hole’s central region, or “shadow,” surrounded by a ring-like structure. That ring is actually light escaping the hot gas that is swirling around the black hole. Light that is too close to the black hole—close enough to be swallowed by it—eventually crosses its horizon, leaving behind just the dark void in the center.

The 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 pictures look remarkably similar because they are the consequence of fundamental force gravity, but in actuality, they couldn’t be more different.

“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,” said Sera Markoff, co-chair of the EHT Science Council and a professor of theoretical astrophysics at the University of Amsterdam. “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.”

M87* is 1,500 times more massive than Sag A*, 2,000 times further away from us, and is accumulating matter at a significantly faster rate than our black hole. Additionally, M87 launches a powerful jet that extends as far as the edge of that galaxy, while Sag A* does not. But despite the “closeness” of Sag A*, capturing an image of the Milky Way’s black hole was considerably more difficult than for M87*.

EHT scientist Chi-kwan Chan likened snapping a picture of Sag A* to “trying to take a clear picture of a puppy quickly chasing its tail.”

“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 M87*, it completes an orbit in mere minutes in the smaller Sgr A*. This means the brightness and pattern of the gas around Sgr A* was changing rapidly as the EHT collaboration was observing it,” explained Chan.

To image the black hole, over 300 researchers from 80 institutes around the world created the 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, similar to using a long exposure time on a camera.

Even with the powerful EHT, the researchers had to develop sophisticated new tools to account for Sag A*’s extraordinary gas movement, including what Ozel thinks may be the most complex imaging algorithms in the world. The scientists 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.

Ultimately, the image of Sgr A* is an average of all the different images the team extracted.

“The light we captured from material swirling around the black hole was changing in brightness on minute timescales, therefore it was critical that we characterized this variability before we could properly calibrate the data and create an image,” said Alexandra Tetarenko, one of the lead coordinators of the Time Domain Working group of the EHT.

The EHT scientists are particularly excited to have images of two black holes of very different sizes, which offers the opportunity to compare and contrast. The team has already begun to use the new data to test theories and models of how gas and gravity behave around supermassive black holes, how giant black holes interact with their surroundings, and how well Einstein's Theory of General Relativity holds up. Thus far, the researchers have been “stunned” by the accuracy of Einstein’s predictions.

“Now we have two laboratories in the sky—M87 and Sag A*—for exquisite tests of extreme environments. We’ve learned so far that we understand gravity pretty well. But as sophisticated as our simulations are, we discovered we still have a way to go with modeling their turbulent environments,” said Ozel.

 

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