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Zitat

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12 May 2022
Black hole at the centre of our Galaxy imaged for the first time

The Event Horizon Telescope network has captured the second-ever direct image of a black hole — called Sagittarius A* — at the centre of the Milky Way.

Radio astronomers have imaged the super massive black hole at the centre of the Milky Way. It is only the second-ever direct image of a black hole, after the same team unveiled a historic picture of a more distant black hole in 2019.

The long-awaited results, presented today by the Event Horizon Telescope (EHT) collaboration, show an image reminiscent of the earlier one, with a ring of radiation surrounding a darker disk of precisely the size that was predicted from indirect observations and from Albert Einstein’s theory of gravity.

“Today, right this moment, we have direct evidence that this object is a black hole,” said astrophysicist Sara Issaoun of the Harvard Smithsonian Center for Astrophysics at a press conference in Garching, Germany.
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During five nights in April 2017, the EHT collaboration used eight different observatories across the world to collect data from both the Milky Way’s black hole — called Sagittarius A* after the constellation in which it is found — as well as the one at the centre of the galaxy M87, called M87*.

But the Sagittarius A* data were more challenging to analyse. The two black holes have roughly the same apparent size in the sky, because M87* is nearly 2,000 times farther away but also roughly 1,600 times larger. This also means that any blobs of matter that spiral around M87* are covering much larger distances — larger than the orbit of Pluto around the Sun — and the radiation they emit is essentially constant over short time scales. But Sagittarius A* can change quickly even over the few hours the EHT observes it every day. “In M87* we saw very little variation within a week,” says Heino Falcke, an astrophysicist at Radboud University in Nijmegen, the Netherlands and a co-founder of the EHT collaboration. “Sagittarius A* varies on time scales of 5 to 15 minutes.”

Because of this variability, the EHT team generated not one image of Sagittarius A*, but thousands, and the image unveiled today is the result of a lot of processing. “By averaging them together we are able to emphasize common features,” said EHT member José Gómez of the Andalusian Institute of Astrophysics in Granada, Spain. The next aim of the project is to generate a movie of the black hole to learn more about its physical properties, said Feryal Özel, an astrophysicist at the University of Arizona in Tucson.

The EHT team conducted supercomputer simulations to compare with their data, and concluded that Sagittarius A* is probably rotating along an axis that roughly points along the line of sight to Earth. The direction of that rotation is anticlockwise, Gómez said.

The source turned out to be unusually dim, dimmer than an average star. Still, decades-long observations of the motions of nearby stars revealed that the object was extremely massive. The most recent ones have measured it to be 4.15 million times the mass of the Sun, give or take 0.3%. These calculations, done by tracking how stars orbit Sagittarius A*, provided strong evidence that the radio source is so massive and dense that it could be nothing else than a black hole, and earned Andrea Ghez and Reinhard Genzel a share of the 2020 Nobel Prize in Physics. (The EHT image shows that the black hole weighs around 4 million solar masses, which is consistent with those earlier estimates, although not as precise.)

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https://www.nature.com/articles/d41586-022-01320-y

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