As difficult as getting a sharp image of a puppy chasing its tail, one researcher describes the challenge of clearly imaging the rapidly changing environment of the black hole at the heart of our Milky Way. And yet the sensation has now succeeded.
How do you photograph an object that is inherently invisible? 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.
The answer: You don’t record the invisible object itself, but its immediate surroundings – and thus make it visible as a dark center in a glowing ring. An international team of researchers has now achieved this coup for the second time – this time with the black hole at the center of our home galaxy, the Milky Way.
“We have reached the next level,” said Anton Zensus from the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn, one of the main initiators of the EHT project. “I’m proud of our entire global team.” For the recording, the largest radio telescopes in the world were combined into a single camera the size of the earth.
A comparison with computer models shows, among other things, that the black hole rotates, the scientists report in a special issue of the specialist magazine “Astrophysical Journal Letters”.
Eight radio telescopes on four continents were linked together for the recording. Together they form the Event Horizon Telescope (EHT). Scientists call the event horizon the limit around a black hole beyond which you cannot see – because nothing, not even light, can escape from the area behind it.
The data from the telescopes are combined with special supercomputers, resulting in a gigantic virtual telescope with the diameter of the earth. It has a level of detail that would allow an orange to be identified on Earth from the moon, as the researchers involved once explained. Or read a newspaper in New York from Berlin.
The international ALMA observatory in Chile, which consists of 66 individual antennas, was involved in the measurements. Also present was 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, the EHT researchers – a total of around 80 institutes with 300 scientists are involved – were able to carry out their first observations with the telescope network in 2017. After the complicated evaluation of the data, the team presented the first photo 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, some 55 million light-years away. The mass of the black hole is enormous: it corresponds to the mass of 6.5 billion suns.
But in April 2017, the researchers not only aimed the many radio antennas of the EHT at this distant galaxy, but also at the center of the Milky Way, which is much closer at 27,000 light years and also contains a massive black hole.
But even though this object, called Sagittarius A*, is much closer to Earth, interpreting the observational data proved far more difficult. “The radiation from the black hole of M87 is constant for hours,” explained Anton Zensus from the MPIfR in Bonn. “The object in the galactic center, on the other hand, changes over the course of just a few minutes. We therefore had to develop completely new methods for the evaluation.”
In both cases, the gas near the black holes moves almost as fast as light, explained EHT scientist Chi-kwan Chan of the Steward Observatory in the USA. It still takes days to weeks to orbit the much larger black hole in M87 – but only a few minutes for the much smaller black hole in the Milky Way. Brightness and patterns in the environment changed quickly accordingly. “It’s a bit like trying to get a sharp picture of a puppy quickly chasing its tail.”
Five years after the observations, the astronomers can finally present the result – the first photo of the black hole at the center of the Milky Way. As with M87, a luminous ring appears around a dark core. The researchers call this dark area the “shadow” of the black hole – it is about twice as large as the actual event horizon because the light is directed around the black hole by the strong gravity and thus towards both the front and the back of the object see are.
The glowing ring is heated gas swirling around the black hole, known as the accretion disk. Gravity also forces the radiation emanating from this gas into 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: The resulting photo is therefore in very good agreement with the expected distortion for a black hole with four million times the mass of the sun.
Again, this value agrees well with previous measurements based on the motion of stars around the black hole. And the exact comparison with different models allows further conclusions. “The models that assume a rotation of the black hole fit best,” says Karl Schuster from the Institute for Millimeter Wave Radio Astronomy in France. “In addition, the axis of rotation of the black hole seems to be more or less tilted towards Earth,” the researcher continues. This is unusual because it does not coincide with the axis of rotation of the Milky Way.
For the EHT researchers, the photo of the galactic center is a great success, but it is only a first step: “It shows us that our method works,” says Zensus. For the future, the researcher hopes for the expansion of the EHT network – if possible also with antennas in space. This would then allow images to be obtained with an even higher resolution – and it is hoped that completely new insights into the physical processes in the immediate vicinity of supermassive black holes could be gained.
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