An estimated 200 billion stars shine in our home galaxy, the Milky Way. There are also a lot of planets, moons and asteroids, all of which rotate around a cosmic axis of rotation. Everything is held together by gravity: the mass effect forces neighboring celestial bodies on course and prevents them from escaping.
For the galactic carousel to work, researchers argue, there must be a very massive black hole at its center. In recent years, they have collected a number of indications of this gravitational giant called Sagittarius A*, Sgr A* for short – such as observations of stellar movements in its immediate vicinity.
An international research team from the Event Horizon Telescope (EHT) is now presenting a photographic evidence. It was featured at several press conferences around the world, as well as in a special issue of the Astrophysical Journal Letters.
The brightly colored image shows the hot matter rotating around the black hole, emitting radiation as it does so. Where it is particularly bright, there is a lot of matter. The hole itself cannot be seen because no radiation comes out that could be measured from Earth. But it can be guessed at as the dark spot around which matter swirls.
Imaging black holes and their surroundings is very expensive. So far, this has only been achieved once, with the specimen in the heart of the galaxy M87. The EHT collaboration published the image three years ago. It has been featured on the front pages and in the main TV news worldwide and has long since become an icon in astrophysics. The portrait of Sgr A* looks very similar to this one, although there are inherent differences between the two objects.
The black hole in M87 is 53 million light-years away and has around 6.5 billion solar masses. Sgr A* is much closer at 27,000 light-years and a lightweight at 4.3 million solar masses. “We have two very different galaxies and two different black holes, but somehow they look very similar when you get close enough to look at the edge of these black holes,” says Sera Markoff, professor of theoretical astrophysics at the University of Amsterdam and Co-Chair of the EHT Science Council. “At this small scale, the complexity surrounding these black holes fades away, revealing these simple objects predicted by general relativity.”
Mariafelicia de Laurentis from the University of Naples, who is part of the EHT team, is also enthusiastic: “It is impressive how well the size of the ring agrees with predictions from Einstein’s general theory of relativity.” understand gravity much better overall.
When expectations are met so well, skepticism is appropriate. Especially since the evaluation of the measurement data was even more difficult than with the M87. There, the researchers were dealing with a real object of the mind. The gas around it takes days to weeks to orbit the black hole once. Hardly anything moved during the observation period from April 4th to 14th, 2017. Sgr A* was observed at the same time. However, it is much lighter, the orbit of matter is smaller, and it takes only minutes to orbit. The picture threatened to blur.
In addition: “We are looking into the disc of the Milky Way,” says Anton Zensus from the Max Planck Institute for Radio Astronomy in Bonn (MPIfR) and founding chairman of the EHT Collaboration Council. “There is a lot of matter on which the radiation is scattered on its way to earth.” The result is a wavering, flickering image.
What’s more, the image is not based on data from a radio telescope, but from eight devices that are distributed on several continents and measured simultaneously. Their data is then combined. The researchers thus have a virtual telescope the size of the earth (see graphic). This method is called Very Long Baseline Interferometry, or VLBI for short.
Only then does the EHT array achieve a resolution with which the relatively small black holes and their surroundings can be identified. In 2019, the EHT researchers chose an orange as a measure of the outstanding angular resolution of their method, which could theoretically be seen on the moon from Earth. This time it’s a donut. Or, as the presentation in Garching said: You could see bubbles in a glass of beer in New York from here.
Several teams independently analyzed the data, testing their results over and over again, reports Zensus. This makes him so sure that the result is robust, that the image depicts the reality in the center of the Milky Way. It’s not really a snapshot, but a composition of many individual images: Regions in which matter was often detected during the series are lighter, the other areas are correspondingly darker.
As beautiful as the picture is, the around 300 scientists involved didn’t just want to create a poster motif. But better understand what is happening there at Sgr A*. “We see that the origin of the submillimeter radiation we detected is in a specific region around the black hole,” says Zensus. “This is important for colleagues who observe in other wavelengths.”
Only the linking of different methods, the so-called multi-messenger astronomy, makes it possible to research and understand the physical processes in depth. It’s about jets, for example, streams of plasma that are thrown out into space perpendicular to the rotating disc of matter. How they form has not yet been clarified, but strong magnetic fields apparently play a role. Further measurements, also with an improved EHT, could help here.
The first steps towards the upgrade have been taken. In the meantime, three more radio telescopes in France, Arizona and Greenland have joined the EHT network. Every year in April they measure together. The only exception was 2020, the campaign had to be canceled due to the Covid 19 pandemic. Three more plants could follow, including one in Africa. This is geographically important for the observations and would also be a driver for the development of science on the continent.
According to the census, there was also the idea of integrating another telescope via the Russian Academy of Sciences. Because of the known sanctions, this will not be pursued for the time being. In the long term, the Bonn researcher would like to go into space anyway. With a radio telescope on a satellite that orbits the earth in a wide arc and complements the facilities down here, the virtual telescope could be significantly enlarged. “That would lead to significantly better data and images.”
The researcher says the idea has already been presented to the European space agency Esa. Even if she could be convinced quickly, such a satellite would hardly be launched in the next two decades.