Astronomers published the first image of a black hole in the heart of our galaxy

At simultaneous press conferences around the world, including at the headquarters of the European Southern Observatory (ESO) in Germany, astronomers published the first image of a supermassive black hole in the center of our own galaxy, the Milky Way. This result not only gives us indisputable evidence that the target is in fact a black hole, but also valuable clues as to the actions of these giants, which are believed to be in the middle of most galaxies. The image was created by the international research team Event Horizon Telescope Collaboration (EHT) on the findings of a global radio telescope network.

This image gives us a long-awaited look at the massive object in the middle of our galaxy. Scientists had already observed the stars orbiting something invisible, compact and very massive in the center of the Milky Way. This fact strongly suggested that this object, known as Sagittarius A * (Sgr A *), was a black hole, and today’s image gives us the first direct visual proof of this.

Although we ourselves do not see a black hole because it is completely dark, the glowing gas surrounding it reveals an unmistakable signature: a dark central area (called a shadow) surrounded by a glowing ring structure. This new image captures the refraction of light by enormous gravity from a black hole that is about four million times more massive than our sun.

We were amazed to see how the size of the ring we observed is so in line with the predictions of Einstein’s general theory of relativity,“said EHT project researcher Geoffrey Bower of the Academia Sinica Institute of Astronomy and Astrophysics in Taipei.”These unprecedented observations have greatly increased our knowledge of what is happening in the center of our galaxy and give us new insights into how these giant black holes interact with their environment.

The results of the EHT Group will be published today in a special issue of the specialty magazine The Astrophysical Journal Letters.

Because the black hole is located about 27,000 light-years from Earth, it looks to us in the sky about the same size as a monk on the Moon. To observe it, the team created a powerful EHT that connects eight radio observatories around the planet together to form one “Earth-sized” virtual telescope. The EHT monitored Sgr A * in 2017 over several evenings, collecting data for several hours in a row in a process similar to long-exposure camera photography.

The network of EHT radio observatories includes, among other infrastructure, the Atacama Large Millimeter / Submillimeter Array (ALMA) and the Atacama Pathfinder EXperiment (APEX), both located in the Atacama Desert in Chile, jointly owned and operated by ESO. For the benefit of the Member States of the European Union. Europe has also participated in EHT observations with other radio observatories – IRAM 30-meter Telescope in Spain and since 2018 NOEMA (NORthern Extended Millimeter Array) in France – and a supercomputer that integrates EHT data and is the Max Planck Institute for Radio Astronomy in Germany. In addition, Europe participated in the EHT consortium project through grants from the European Research Council and the German Max Planck Society.

It is very gratifying that ESO has played such an important role in solving the mysteries of black holes, and Sgr A * in particular, for so many years,”Commented Xavier Barcons, CEO of ESO. “ESO not only participated in the EHT observations through ALMA and APEX, but also, together with its other Chilean observatories, enabled some previously very successful observations of the galaxy center.” [2]

In fact, this EHT result follows that in 2019, the Collaboration published the first image of a black hole, M87 *, located in the center of a more distant galaxy, Messier 87.

The two black holes are very similar, despite the fact that the black hole in our Galaxy is about a thousand times smaller and less massive than the M87 * [3]. “We have two completely different types of galaxies and two black holes with very different masses, but at the boundary of these black holes the similarities are surprisingly large.Says Sera Markoff, President of the Scientific Council of the EHT and Professor of Theoretical Astrophysics at the University of Amsterdam in the Netherlands. “This fact tells us that general theory of relativity dominates these objects at their core and that all the differences we see further away are due to differences in the material surrounding black holes.

This result was considerably more difficult to obtain than the M87 *, although Sgr A * is much closer to us. Chi-Kwan (‘CK’) Chan, an EHT researcher at the Steward Observatory and the Department of Astronomy and Scientific Data at the University of Arizona, explains: “The gas near the black holes travels at the same speed – almost the speed of light – around both Sgr A * and M87 *. However, the gas goes from days to weeks to orbit the much larger M87 *, while around the lower Sgr A * it orbits in a matter of minutes. As a result, the brightness and pattern of the gas surrounding the Sgr A * varied rapidly as the EHT Collaboration observed it – a bit like trying to take a clear picture of a dog chasing its tail at full speed.

Researchers had to develop advanced new tools that take into account the movement of gas around Sgr A *. Although the M87 * was a lighter and more stable target, almost all the images obtained looked quite similar, this no longer applies to the Sgr A *. The image of the black hole Sgr A * is the average of several images taken by the group that will eventually reveal to us and for the first time the giant lurking in the middle of our galaxy.

This work was made possible by the collaboration of more than 300 researchers from about 80 institutions around the world. They joined the EHT cooperation. In addition, in developing complex tools to address the imaging challenges of Sgr A *, the team worked hard for five years using supercomputers to combine and analyze data while assembling an unprecedented library of simulated black holes to compare to observations.

Scientists are particularly excited to finally get images of two very different sized black holes, allowing us to compare and contrast them. The team has also begun to use new data to test theories and models of gas behavior around supermassive black holes. Although this process is not yet fully understood, it is believed to play a crucial role in the formation and evolution of galaxies.

We can now explore the differences between these two supermassive black holes to gain new insights into how this important process works.“said Keiichi Asada, an EHT researcher at the Academia Sinica Institute of Astronomy and Astrophysics in Taipei.”We have images of two black holes – the big and the small – so we can go much further than before to understand the behavior of gravity in these extreme environments.

The progress of the EHT continues: a massive observation campaign in March 2022 included more telescopes. The continuous expansion of the EHT network and major technological upgrades will allow astronomers to obtain more impressive images and films of black holes in the near future.

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