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May 16, 2022
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Astronomers today released the first ever image of the supermassive black hole at the center of the Milky Way Galaxy—or an image of its shadow at least. This image, of the object known as Sagittarius A* (Sgr A), thought impossible just a few years ago, was achieved through the combined efforts of eight radio observatories around the globe and more than 300 scientists worldwide. “Until now, we didn’t have the direct picture confirming that Sgr A was indeed a black hole,” team member Feryal Özel of the University of Arizona told a press conference in Washington, D.C., today.

The team, known as the Event Horizon Telescope (EHT), in 2019 produced the first ever image of a black hole, at the center of the nearby giant galaxy M87. The two images look remarkably similar and that is part of what is extraordinary about it, researchers say. The M87 black hole is 1600 times more massive than Sgr A* and lies at the heart of a much larger galaxy. Yet the similarity of the two images—bright rings of gas trapped in death spirals around these ultimate sinkholes—demonstrates that Albert Einstein’s theory of gravity, general relativity, works the same at all scales. Despite the different environments, once you get close to the black hole “gravity takes over,” says EHT team member Sara Issaoun of the Harvard & Smithsonian Center for Astrophysics (CfA). Getting this second image shows it wasn’t a coincidence, Özel says. “We now know that in both cases, what we see is the heart of the black hole, the point of no return.”

Compared with M87, which converts swirling gas into a powerful jet thousands of light-years long, Sgr A* appears to be quiet. “M87 was exciting because it was extraordinary,” says CfA’s Michael Johnson. “Sgr A* is exciting because it’s common.” Initial analysis of the new image suggests only a trickle of gas makes it to the black hole, and only one part in 1000 is being converted to light, Johnson says. “The black hole is ravenous but inefficient.” Simulations also suggest the black hole is spinning, although the team doesn’t see direct evidence for spin in the image, Johnson adds.  

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Although supermassive black holes have enormous masses—millions or billions of times that of the Sun—their intense gravity means that their outer edge, the event horizon, is tiny in galactic terms. Sgr A*, which has a mass of 4 million Suns, has an event horizon that is just 15 times the size of the Earth-Moon distance. Imaging something so small from 27,000 light-years away presents a huge challenge for astronomers.

The first challenge is dust: Clouds of dust around the galactic center make observations with optical telescopes impossible. Radio telescopes can peer through the dust, but their long wavelengths don’t offer the resolution to spot a diminutive black hole. There’s a sweet spot at the shortest radio wavelengths of about 1 millimeter where light can pierce the gloom and offer a sharp enough image. Telescopes that observe at that wavelength are a relatively new breed. Unlike normal radio telescopes, they must be built at high-altitude sites to get above most of the moisture in Earth’s atmosphere.

In astronomy, the bigger the telescope, the sharper the image. Astronomers calculated decades ago that to see Sgr A* in millimeter waves, a telescope aperture as wide as Earth would be needed. Such a telescope not being available, the EHT does the next best thing: It observes the galactic center with a collection of telescope dishes scattered across the globe at the same time. The EHT team stores the data, and later processes it with powerful computers as if each dish was a small patch of an Earth-wide aperture—a technique known as very long baseline interferometry (VLBI). “Each pair [of telescopes] contributes a little bit of information to the entire image,” says EHT team member Katie Bouman of the California Institute of Technology.

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VLBI had not been attempted before in millimeter waves so in the 2010s, the EHT team had to develop new observing and processing techniques and adapted a handful of dishes to see whether it could work. By 2017 the team was ready to take a shot at Sgr A* and the nearby giant galaxy M87 using eight observatories, from Hawaii to Spain and from Arizona to the South Pole. A key addition was the Atacama Large Millimeter/submillimeter Array in Chile, a group of 64 dishes that together act like an 84-meter-wide telescope.  

Processing and interpreting the data took longer than expected and, because of the new methods, the team was careful in interpreting the image. “It was the best vetted image in radio astronomy ever,” says EHT team member Heino Falcke of Radboud University. In April 2019, the team released its now famous image of M87, a result chosen as Science’s 2019 Breakthrough of the Year.

M87 was processed first. Sgr A* proved a harder nut to crack, first because the telescopes were viewing it through the crowded central plane of the Milky Way, where electrons from ionized gases scattered the light. Johnson describes it as peering through “frosted glass.” The second challenge was motion. Gas moves slowly around M87’s giant black hole, taking days to orbit the event horizon. But for the much smaller Sgr A*, gas takes anywhere from 4 minutes to 1 hour to orbit so during an observation lasting several hours there is a lot of movement. “If an object changes in a crazy way, you can’t image it with VLBI,” Falcke says.

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The three “knots” visible in the image could be expected from natural variations in the bright swirling gas, but Özel says they could also be artifacts of observation process. “We don’t trust the knots that much,” she says. Still, Falcke says after 2 years scrutinizing the quality of the results, the team is confident the ring of light surrounding the black hole shadow represents reality. “On top of the chaotic structure you have a stable structure,” he says.

Unlike for M87, the mass of Sgr A* is known very precisely from studies of star orbits close to the black hole, so the team had a firm idea of what it should be seeing. “It’s a very tight prediction, with no wiggle room,” Falcke says.

The EHT team has an enormous backlog of observations to analyze. It carried out further observing campaigns in 2018, 2021, and 2022 and will soon begin to process data from them. “Now that the tools are ready, we hope it will be faster,” Özel says. Since 2017, the EHT collaboration has added new dishes in Greenland, France, and the United States, and it hopes to soon begin to build another in Namibia. In the future, the researchers plan to start to observe at a shorter wavelength—0.86 millimeters, compared with the 1.3 millimeters used so far—which will allow them to see even closer to the event horizon. Another aim is to make movies of motion around the black hole with regular observations. Observing M87 every 2 weeks is the first goal. Later, they will try for Sgr A*-The Movie. “You need more observations at the same time,” Falcke says. “It’s a lot of hard work.

This story will be updated.

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