It has remained hidden for a long time, but is now becoming visible for the first time: astronomers have managed to take the first image of Sagittarius A *, a black hole at the center of our Milky Way. The Event Horizon image shows a dark shadow of a supermassive black hole surrounded by a bright ring of rays of diffracted light. The photo therefore provides the first direct visual evidence of the existence of this black hole and confirms Einstein’s prediction of the appearance of such black holes. In addition, the image now provides preliminary comparisons with the M87 * black hole, which was recorded for the first time in 2019 and is about a thousand times larger than the Sagittarius A *.
It has long been known that the supermassive black hole is at the center of almost all galaxies. Such a gravitational giant also hides at the heart of our Milky Way – at least that’s what indirect observations suggest. Consequently, the black hole, which has a mass of about four million times the mass of the Sun, affects the orbits of countless stars at the center of the Milky Way, and also stretches their light, as observations show. From the movements of the stars and gases, astronomers were able to determine its position and mass, as well as obtain 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. Worse, although 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. So even the most powerful telescopes could not see much.
Hidden giant in sight
That has changed now thanks to the Event Horizon Telescope (EHT) network. Because this global fusion of eight radio telescopes forms a virtual antenna dish almost the size of the Earth. The resulting high resolution allowed EHT astronomers to capture the Sagittarius A * radiation signature at the center of the Milky Way. To do this, they targeted a black hole for several nights in 2017. However, creating an image from observational data was much more difficult for Sagittarius A * than for the 1,000-fold larger M87 * black hole photographed in 2019.
The reason is the rapid movement of the glowing plasma around our “native” black hole: “While gas takes days or weeks to orbit the larger M87 *, it orbits 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 observation. “It’s a bit like trying to get a clear picture of a puppy chasing its tail quickly,” says Chan. In order to create a picture, astronomers had to create a kind of average of all the pictures taken, which required enormous computational effort and the use of special analytical tools. Over 300 scientists from 80 institutes around the world were involved in these five-year work.
Dark shadow and light ring
The resulting image shows for the first time the appearance of a gravitational giant at the heart of our galaxy. Inside, you can see the dark shadow of a black hole – a zone from which even light can no longer escape. The shadow surrounds a bright ring of radiation that is held and flexed by the black hole’s gravity. Albert Einstein had already predicted such ring formation for black holes. “We were amazed at how well the ring size matched the predictions of Einstein’s general relativity,” said Geoffrey Bower of the Institute of Astronomy and Astrophysics at Academia Sinica in Taipei. The image of Sagittarius A * provides the first direct visual evidence that the object at the center of the Milky Way is actually 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 insight into how these giant black holes are connected to their environment.”
At the same time, it allows for the first comparisons with the supermassive black hole M87 *. It lies at the center of a distant galaxy and is much more massive and about a thousand times larger than Sagittarius A *. “We now have images of two black holes – one at the high end of the mass spectrum and the other at the low end,” says Bower’s colleague Keiichi Asad. Comparing the two black holes now allows astronomers to study 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 insight into how this important process works,” said Asada.
The new image already shows 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 appear.
Source: Cooperation under the Event Horizon program, European Southern Observatory