It remained hidden for a long time, but now it is becoming visible for the first time: astronomers have succeeded in taking the first photo of Sagittarius A*, the black hole in the center of our Milky Way. The Event Horizon telescope image shows the dark shadow of the supermassive black hole surrounded by a bright ring of rays of diffracted light. The photo thus provides the first direct visual evidence for the existence of this black hole and confirms Einstein’s predictions about the appearance of such black holes. In addition, the image now allows initial comparisons with the black hole M87*, which was recorded for the first time in 2019 and is around a thousand times larger than Sagittarius A*.
It has long been known that a supermassive black hole sits at the center of almost all galaxies. Such a gravitational giant is also hidden in the heart of our Milky Way – at least that is what indirect observations suggest. Accordingly, the black hole, which has a mass of around four million solar masses, influences the orbits of countless stars in the center of the Milky Way and also stretches the light they emit, as observations show. From the motions of stars and gases, astronomers have been able to determine its position and mass, as well as gain early clues about other features such as rotation and magnetic fields. However, because the black hole is rather inactive and does not absorb large amounts of matter, Sagittarius A* itself remained invisible. To make matters worse, while the black hole is “only” 27,000 light-years away, it is hidden behind dense dust and stars. In addition, from our perspective, its event horizon is only the size of a tennis ball on the moon. Therefore, even the most powerful telescopes could not see much.
Hidden giant in sight
That has now changed thanks to the Event Horizon Telescope Network (EHT). Because this global merger of eight radio telescopes forms a virtual antenna dish almost the size of the earth. The resulting high resolution allowed astronomers from the EHT collaboration to capture the radiation signature of Sagittarius A* at the center of the Milky Way. To do this, they targeted the black hole for several nights in 2017. However, creating the photo from the observation data was significantly more difficult for Sagittarius A* than for the thousand times larger black hole M87* photographed in 2019.
The reason is the fast movement of the glowing plasma around our “native” black hole: “While the gas takes days to weeks to orbit the larger M87*, it completes one orbit around the much smaller Sgr A* in just a few minutes” , explains Chi-kwan Chan of the University of Arizona. As a result, the brightness and pattern of the gas around Sagittarius A* constantly changed during the observation. “It’s a bit like trying to get a clear picture of a puppy quickly chasing its tail,” says Chan. In order to create the photo, the astronomers therefore had to create a kind of average of all the images taken, which involved enormous computational effort and the use of special analysis tools. More than 300 researchers from 80 institutes around the world were involved in this five-year work.
Dark shadow and light ring
The resulting image shows for the first time the appearance of the gravity giant at the heart of our galaxy. In the middle you can see the dark shadow of the black hole – the zone from which even light can no longer escape. Surrounding the shadow is a bright ring of radiation that is being held and diffracted by the black hole’s gravity. Albert Einstein already predicted such ring formation for black holes. “We were amazed at how well the size of the ring matched the predictions of Einstein’s general theory of relativity,” said Geoffrey Bower of the Institute of Astronomy and Astrophysics at Academia Sinica in Taipei. The image of Sagittarius A* thus provides the first direct visual evidence that the object at the center of the Milky Way is in fact a black hole. “These unprecedented observations have greatly improved our understanding of what is happening at the center of our galaxy,” says Bower. “They offer new insights into how these giant black holes are connected to their environment.”
At the same time, it allows first comparisons with the supermassive black hole M87*. This lies at the center of a distant galaxy and is far more massive and around a thousand times larger than Sagittarius A*. “We now have images of two black holes — one at the top and one at the bottom of the mass spectrum,” says Bower’s colleague Keiichi Asada. Comparing both black holes now allows astronomers to explore and understand the gravitational effects of such gravitational giants better than before. “Now we can study the differences between these two supermassive black holes to gain valuable new insights into how this important process works,” Asada said.
The new image already reveals that the two black holes look remarkably similar despite their size difference. “This tells us that general relativity dominates at close range for these objects,” explains Sera Markoff of the University of Amsterdam. Only at greater distances from the event horizon do differences in the amount and type of surrounding material come into play.
Source: Event Horizon Collaboration, European Southern Observatory