The first image of a black hole at the center of the Milky Way


The first image of a black hole at the center of the Milky Way

The first image of Sagittarius A*, the black hole at the center of our galaxy, about 27,000 light-years from Earth.

picture: dpa

The first image of a black hole caused a sensation three years ago. Researchers now provide an image of the black hole at the center of the Milky Way. This requires completely new methods.

Bonn. How do you depict an object that is invisible by nature? This problem has plagued astronomers since the first theoretical speculations about black holes – objects whose gravitational pull is so violent that not even light can escape from them.

Answer: You do not record the invisible object itself, but its immediate surroundings – thus making it visible as the dark center of a glowing ring. An international research team has now succeeded in this reversal a second time – this time with the black hole at the center of our Milky Way.

“What could be cooler than seeing a black hole at the center of our Milky Way?” Katie Bowman, a computer scientist at the California Institute of Technology, said at a news conference in the US.

Earth Size Camera

“We have reached the next level,” said Anton Zonsus of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, one of the main initiators of the EHT project. “I am proud of our entire global team.” For the record, the world’s largest radio telescopes were combined into a single Earth-sized camera. Vincent Fish of the MIT Haystack Observatory in the US explained that the telescopes collected about three and a half petabytes of data – the equivalent of about 100 million TikTok videos. “It’s way too much data is streamed over the internet. We actually have to ship our hard drives all over the place.”

Comparison with computer models showed, among other things, that the black hole rotates, as scientists reported in a special issue of the specialized journal “Astrophysical Journal Letters”. Eight radio telescopes on four continents were linked together for the recording. Together they make up the Event Horizon Telescope (EHT). Scientists call the event horizon the boundary around the black hole beyond which you can’t see – because nothing, not even light, can escape from the area behind it.

Data from telescopes is combined with special supercomputers, resulting in a giant virtual telescope with the diameter of the Earth. It contains a level of detail that would allow an orange to be identified on Earth from the Moon, the participating researchers once explained. Or read a newspaper in New York from Berlin.

80 institutes with the participation of 300 scientists

The ALMA International Observatory in Chile, which consists of 66 individual antennas, took part in the measurements. He also attended the German-French-Spanish Institute for Radio Astronomy in the Millimeter Range (IRAM), which works with the 30-meter telescope in Spain and the NOEMA interferometer in France.

After years of preparatory work, EHT researchers – a total of about 80 institutions with 300 scientists participating – were able to make their first observations using the telescope’s network in 2017. After complex evaluation of the data, the team provided the first image of a black hole – or, more precisely: its immediate surroundings – In 2019. The image shows a glowing ring around the supermassive black hole at the center of galaxy M87, about 55 million light-years away. The mass of the black hole is enormous: it corresponds to the mass of 6.5 billion suns.

‘Totally New Roads’ Required for Sagittarius A*

However, in April 2017, researchers not only point the EHT’s numerous radio antennas at this distant galaxy, but also at the center of the Milky Way, which is much closer to 27,000 light-years away and also contains a supermassive black hole. But although this object, called Sagittarius A*, is much closer to Earth, interpretation of the observational data has proven more difficult. “The radiation from M87’s black hole is constant for hours,” explained Anton Zonsus of MPIfR in Bonn. “On the other hand, the object in the galactic center changes over the course of a few minutes. So we had to develop completely new methods of evaluation.”

In both cases, gas near black holes moves at nearly the speed of light, explained EHT scientist Chi Kuan Chan of the Steward Observatory in the USA. It still takes days to weeks to orbit the much larger black hole of M87 – while the Milky Way’s much smaller black hole takes just a few minutes. Brightness and patterns in the environment change rapidly accordingly. “It’s a bit like trying to get a sharp picture of a puppy chasing its tail fast.”

The picture matches Einstein’s predictions

After five years of observations, astronomers can finally provide the result – the first image of a black hole at the center of the Milky Way. As with the M87, a luminous ring appears around a dark core. Researchers call this dark region the black hole’s “shadow” – it’s twice as large as the actual event horizon, because light is deflected by strong gravity around the black hole, and thus toward both the front and back of the black hole. see the object.

The glowing ring is hot gas orbiting the black hole, known as an accretion disk. Gravity also forces radiation from this gas onto curved paths, creating a distorted view of the black hole’s surroundings.

With the help of computer models, the scientists compared their observations with the predictions of Albert Einstein’s general theory of relativity about black holes: so the image obtained is in very good agreement with the expected distortion of a black hole with a mass four million times the mass of black holes. the sun. “We were surprised by how well the size of the ring matched the predictions of Einstein’s general theory of relativity,” said EHT scientist Jeffrey Bauer of the Institute of Astronomy and Astrophysics at Academia Sinica in Taipei.

Again, this value fits well with previous measurements based on the motion of stars around the black hole. A careful comparison with different models will allow further conclusions. “Models that assume a black hole’s rotation fit better,” says Karl Schuster of the Millimeter Wave Radio Astronomy Institute in France. In addition, the black hole’s rotation axis appears to be somewhat tilted toward Earth. This is unusual because it does not coincide with the rotation axis of the Milky Way.

Just a first step

In the Milky Way, the Sun orbits around Sagittarius A* along with 200 or 300 billion stars and clouds of gas and dust. For the EHT researchers, the image of the galactic center is a huge success, but it’s also only a first step. “We have images of two black holes – one at the top and one at the bottom of supermassive black holes in the universe – which allow us to go further than ever before in studying the behavior of gravity in these extreme environments,” said EHT researcher Keiichi Asada of Academia Sinica. in Taipei.

For the future, the researcher at Bonn Zensus hopes to expand the EHT network, if possible also by using antennas in space. This will then allow for images at significantly higher resolutions, and so we hope that there will be entirely new insights into the physical processes in the immediate vicinity of supermassive black holes. Eleven observatories have already participated in a large EHT campaign in March 2022. The results are eagerly awaited.

© dpa-infocom, dpa: 220512-99-259085 / 5

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