At simultaneous press conferences around the world, including at the European Southern Observatory (ESO) headquarters in Germany, astronomers have released the first image of the supermassive black hole at the center of our galaxy, the Milky Way. This result gives us not only conclusive evidence that the object is in fact a black hole, but also valuable clues about the functioning of these giants, which are believed to be at the center of most galaxies. The image was created by an international team of researchers, the Event Horizon Telescope Collaboration (EHT), from observations taken by a global network of radio telescopes.
This image gives us a long-awaited look at the massive object at the center of our galaxy. Scientists have already noticed that stars orbit an invisible, compact and very massive object at the center of the Milky Way. This fact strongly suggests that this object, known as Sagittarius A* (Sgr A*), was a black hole, and today’s image gives us the first direct visual evidence of this.
Although we do not see the black hole itself, because it is completely dark, the glowing gas that surrounds it reveals an unmistakable signature: a dark central region (called the shadow) surrounded by a glowing ring structure. This new image captures the bending of light under the influence of the massive gravitational force of the black hole, which is about four million times the mass of our sun.
“We were surprised to see how the size of the ring we observed was in line with the predictions of Einstein’s general theory of relativity,said EHT project scientist Jeffrey Bauer of the Institute of Astronomy and Astrophysics at Academia Sinica in Taipei. “These unprecedented observations have greatly increased our knowledge of what is happening at the center of our galaxy and give us new insights into how these giant black holes interact with their surroundings.“
The results of the EHT team were published today in a special issue of the journal Astrophysical Journal Letters.
Since it is located at a distance of about 27,000 light-years from Earth, a black hole appears to us in the sky the same size as a doughnut on the surface of the Moon. To monitor this, the team created a powerful EHT, linking eight radio observatories across the planet together to form a single “Earth-size” virtual telescope. EHT observed Sgr A* in 2017 over several nights, and collected data over many consecutive hours, in a process similar to taking a long-exposure photograph with a camera.
In addition to other infrastructure, the EHT network of radio observatories includes the Atacama Large Millimeter/submillimeter Array (ALMA) and Atacama Pathfinder EXperiment (APEX), both located in the Atacama Desert in Chile, jointly owned by ESO. . for the benefit of its member states in Europe. Europe has also contributed to EHT observations with other radio observatories – the 30-meter IRAM telescope in Spain and, since 2018, NOEMA (NORthern Extended Millimeter Array) in France – as well as a supercomputer that collects EHT data and is at the Max Planck Institute for Radio Astronomy In Germany. In addition, Europe has also contributed to the funding of the EHT Consortium project through grants from the European Research Council and the Max Planck Society in Germany.
“It is with great pleasure that ESO has played such an important role in unraveling the mysteries of black holes, in particular Sgr A*, over many years,ESO General Manager Xavier Barcons commented. “ESO has not only contributed to EHT observations with ALMA and APEX, but has also, with its other observatories in Chile, made possible some previously very successful observations of the galactic center.” 
In fact, this EHT result comes after Collaboration released in 2019 the first image of a black hole, M87*, located at the center of a 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 M87* . “We have two completely different types of galaxies and two black holes with very different masses, however, near the boundaries of these black holes, the similarities are surprisingly huge,says Cera Markov, co-chair of the EHT Scientific Council and professor of theoretical astrophysics at the University of Amsterdam in the Netherlands. “This fact tells us that general relativity essentially governs these things and that any differences we see more are due to differences in the material surrounding black holes.“
Obtaining this result was much more difficult than obtaining M87*, even though Sgr A* is much closer to us. Chi-kwan (“CK”) Chan, EHT Scientist at the Steward Observatory and Department of Astronomy and Scientific Data Institute at the University of Arizona, USA, explains:Gas moves near black holes at about the same speed – roughly the speed of light – around Sgr A* and about M87*. However, gas takes days to weeks to orbit the much larger M87*, while gas around the smaller Sgr A* completes an orbit in just a few minutes. Thus, the brightness and pattern of the gas surrounding Sgr A* varied rapidly as the EHT Collaboration team was observing it—a bit like trying to take a clear picture of a dog chasing its tail at full speed.“
The researchers had to develop sophisticated new tools to calculate the movement of gas around Sgr A*. While the M87* was an easier and more stable target, with nearly all acquired images appearing to be quite similar, this is no longer the case for Sgr A*. The image of the black hole Sgr A* is a number of different images taken by the team, which finally reveal to us, for the first time, the giant lurking at the center of our galaxy.
This work was made possible by the combined efforts of more than 300 researchers from nearly 80 institutions around the world, who joined the EHT Collaboration. Additionally, in developing complex tools to overcome the challenges of Sgr A* imaging, the team has worked rigorously for five years, using supercomputers to integrate and analyze data, while compiling an unprecedented library of simulated black holes for comparison observations.
Scientists are especially excited to finally get two images of two very different black holes, giving us the opportunity to compare and contrast them. The team also began using the new data to test theories and models of how gas behaves around supermassive black holes. Although not yet fully understood, this process is believed to play an important role in the formation and evolution of galaxies.
“We can now study the differences between these two supermassive black holes to gain new insights into how this important process works,“said Keiichi Asada, an EHT scientist at the Institute of Astronomy and Astrophysics at Academia Sinica in Taipei.”We have images of two black holes – a large and a small – so we will be able to go further than before in understanding how gravity behaves in these extreme environments.“
EHT’s progress continues: a large-scale observation campaign carried out in March 2022 included more telescopes than previously. The ongoing expansion of the EHT network and significant technological updates will allow astronomers to obtain even more impressive images, as well as films of black holes, in the near future.
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