Here is the black hole at the center of our galaxy

Scientists have unveiled the first image of the supermassive black hole at the center of our galaxy, the Milky Way. This result is overwhelming evidence that this object is in fact a black hole and provides important clues to understanding the behavior of these bodies that are believed to reside at the center of most galaxies. Obtaining this image, thanks to a global network of radio telescopes, is the Event Horizon Telescope (EHT) Collaboration, an international team that includes researchers from the National Institute of Astrophysics, the National Institute of Nuclear Physics, the Federico II University of Naples and the University of Cagliari.

The long-awaited image finally shows the massive object lurking at the center of our galaxy. Scientists had previously discovered stars moving around an invisible, compact and very massive body at the center of the Milky Way. Those observations suggested that the object in question, called Sagittarius A* (Sgr A*), was a black hole, and the image made public today provides the first direct visual evidence to support this hypothesis.

Although we cannot see the black hole itself, because it does not emit light, the gas shining around it possesses a distinctive appearance: a dark central region (called the black hole's 'shadow') surrounded by a bright ring-shaped structure. The new image captures light distorted by the powerful gravity of the black hole, which has a mass four million times that of the Sun.

"We were amazed at how well the size of the ring agrees with the predictions of Einstein's theory of general relativity," comments Geoffrey Bower, EHT Project Scientist at Academia Sinica in Taipei, Taiwan, and at the University of Hawaii at M'noa in the United States. The results are described in a series of papers published today, May 12, in a special issue of The Astrophysical Journal Letters.

"This is an extraordinary achievement whose magnitude we will only be able to truly realize with time," says Minister of University and Research Maria Cristina Messa. "Congratulations to the large and global team that made it possible to achieve it and, within that, to the Italian scientists. This finding demonstrates how international collaborative research networks are fundamental for the progress of all, how important it is for Italy to be part of them by investing, continuously and stably over the years, in large research and data infrastructures, to strengthen and implement them more and more, and how an effort should be made to preserve these networks even in times of crisis. This result also reminds us that we should not always be in a hurry to reach a given result in a very short time: research has its own timescales and we must have the patience to adapt to these, knowing that it will always be worth it."

The black hole, located about 27 thousand light-years from Earth in the direction of the constellation Sagittarius, appears in the sky with a size equal to what a doughnut on the Moon would be. To make the image, the team created the powerful EHT by bringing together eight radio-astronomical observatories around the world to create a single virtual telescope the size of planet Earth. EHT observed Sgr A* for several nights in April 2017, collecting data for many hours at a time, similar to taking a long exposure with a camera.

Crucial to achieving this result was the contribution of ALMA, the Atacama Large Millimeter/submillimeter Array, the most powerful radio telescope in existence, which from the Atacama Desert in Chile scans the cosmos in the radio band at millimeter and submillimeter wavelengths. Italy participates in ALMA through ESO, the European Southern Observatory, and hosts the Italian node of the European ALMA Regional Center at the headquarters of the National Institute of Astrophysics (INAF) in Bologna.

The discovery comes after the first image of a black hole, the one at the center of the distant galaxy M87, was made public by the EHT Collaboration in 2019. The two black holes appear remarkably similar, although the one in the heart of our galaxy is more than a thousand times smaller and less massive than the one in M87. "We have two completely different types of galaxies and two black holes with very different masses, but near the edge of these black holes, the appearance is strikingly similar," says Sera Markoff, professor of theoretical astrophysics at the University of Amsterdam, Netherlands, and Co-Chair of the EHT Scientific Council. "This tells us that general relativity governs these objects closely, and any differences we see in more distant regions must be due to differences in the material surrounding the black holes."

"The observations provide further support that the spacetime around black holes is described by solutions of general relativity, regardless of their mass," comments Mariafelicia De Laurentis, professor of astrophysics at the University Federico II in Naples and researcher at the National Institute of Nuclear Physics (INFN), Deputy Project Scientist, member of the Scientific Council and coordinator of EHT's Gravitational Physics group, who led the paper on gravity tests. "Studies of the galactic center have allowed many tests of general relativity to be performed over the years, but the result presented today is unprecedented because it allows many original measurements on gravity and to do new science on supermassive black holes and their role in the evolution of the universe: we have opened the door to an extraordinary new laboratory."

Getting the new result was much more difficult than the previous one, even though Sgr A* is much closer to us. The team had to develop sophisticated new data analysis tools to account for the motion of the gas around Sgr A*, which takes only minutes to complete an orbit around this black hole. The black hole at the center of the M87 galaxy is much larger, and the gas, moving at the same speed (close to that of light) around both black holes, takes days or even weeks to orbit around it: it was therefore a more stable target, and almost all the images looked the same. The same was not the case for Sgr A*. The image of the black hole at the center of our galaxy is an average of the different images extracted by the team, finally revealing this object for the first time.

"Variability is one of the critical aspects of Sgr A*: while it poses a major challenge for imaging the galactic center, it also provides us with a fundamental tool for investigating the physical processes taking place there," comments Nicola Marchili, INAF researcher and second author of one of the official papers, who worked on the data analysis of the black hole's temporal variability. "The variability estimated from the EHT data is much lower than expected based on most current theoretical models and therefore places stringent constraints on the physical properties of the black hole," adds Marchili, who works at the ALMA European Regional Center in Bologna together with INAF researchers Elisabetta Liuzzo and Kazi Rygl, who are also part of the EHT Collaboration, within which they are primarily involved in data calibration.

The researchers are excited to finally have images of two black holes of different sizes-an opportunity to understand their similarities and differences. They have also begun using the new data to test the theory and models that describe the behavior of gas around supermassive black holes-a process still not fully understood but believed to be key in the formation and evolution of galaxies in the Universe.

"In addition to developing new tools to image Sgr A*, the team has produced millions of images with different combinations of parameters for the various imaging algorithms, using large computational infrastructures," adds Rocco Lico, INAF associate and researcher at the Instituto de Astrofísica de Andalucía, in Spain, co-leader of one of the groups working on data analysis in the Imaging working group and Information-technology officer of the EHT Collaboration. "In this process, an unprecedented library of simulated black holes was also compiled to compare with observations."

This achievement is the result of the work of more than 300 male and female researchers from 80 institutes around the world who together form the EHT Collaboration.

"Getting this image was always our goal from the beginning of the project, and to be able to reveal it to the world today repays us for many years of hard work," says Ciriaco Goddi, a professor at the University of Cagliari and an INAF and INFN associate, who has been part of this enterprise since 2014 as coordinator of the European BlackHoleCam group, one of the projects from which the EHT Collaboration originated. "The EHT network is continuously expanding and undergoing major technological upgrades-so we will be able to have even more impressive images and even movies of black holes in the near future."

Indeed, EHT's work does not stop: a new observing campaign including three new radio telescopes was conducted last March.