Astronomers have unveiled the first image of the supermassive black hole at the center of our own Milky Way galaxy.
The result provides overwhelming evidence that the object is indeed a black hole and yields valuable clues about the workings of such giants, which are thought to reside at the center of most galaxies. The image was produced by a global research team called the Event Horizon Telescope (EHT) Collaboration, using observations from a worldwide network of radio telescopes.
The image is a long-anticipated look at the massive object that sits at the very center of our galaxy. Scientists had previously seen stars orbiting around something invisible, compact and very massive at the center of the Milky Way. This strongly suggested that this object-known as Sagittarius A* (Sgr A*, pronounced “sadge-ay-star”)-is a black hole, and today’s image provides the first direct visual evidence of it.
Although we cannot see the black hole itself, because it is completely dark, glowing gas around it reveals a telltale signature: a dark central region (called a “shadow”) surrounded by a bright ring-like structure. The new view captures light bent by the powerful gravity of the black hole, which is four million times more massive than our Sun.
“We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity,” said EHT project scientist Geoffrey Bower from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei. “These unprecedented observations have greatly improved our understanding of what happens at the very center of our galaxy, and offer new insights on how these giant black holes interact with their surroundings.”
The EHT team’s results are published in a special issue of the Astrophysical Journal Letters.
Because the black hole is about 27,000 light-years away from Earth, it appears to us to have about the same size in the sky as a donut on the Moon. To image it, the team created the powerful EHT, which linked together eight existing radio observatories across the planet to form a single “Earth-sized” virtual telescope. The EHT observed Sgr A* on multiple nights in 2017, collecting data for many hours in a row, similar to using a long exposure time on a camera.
Mark Reynolds, associate research scientist at the University of Michigan, was involved in the research as part of the multi-wavelength team, a group of scientists making observations at multiple wavelengths, such as radio, infrared, X-ray and gamma ray.
This team ensured that the emission from our Galactic supermassive black hole was observed at all wavelengths at the same time as the EHT. For the 2017 EHT campaign, his team used NASA’s Neil Gehrels Swift Observatory to carry out a dense monitoring campaign-measuring the emission many times during the EHT observation window-of Sgr A* at X-ray wavelengths in order to measure potential X-ray flaring activity from the black hole.
“The resulting dataset will have amazing legacy value to the accretion physics community, while the image of the black hole itself is a sight to behold,” Reynolds said. “To take part in this historic EHT campaign and unveil the supermassive black hole in our Galactic backyard-Sgr A*-is an amazing scientific achievement and a great testament to the brilliance of the human mind.”
The breakthrough follows the EHT collaboration’s 2019 release of the first image of a black hole, called M87*, at the center of the more distant Messier 87 galaxy. The two black holes look remarkably similar, even though our galaxy’s black hole is more than a thousand times smaller and less massive than M87*.
“We have two completely different types of galaxies and two very different black hole masses, but close to the edge of these black holes they look amazingly similar,” said Sera Markoff, co-chair of the EHT Science Council and professor of theoretical astrophysics at the University of Amsterdam, Netherlands. “This tells us that General Relativity governs these objects up close, and any differences we see further away must be due to differences in the material that surrounds the black holes.”
This achievement was considerably more difficult than for M87*, even though Sgr A* is much closer to us. EHT scientist Chi-kwan (CK) Chan, from Steward Observatory and Department of Astronomy and the Data Science Institute of the University of Arizona, explains.
“The gas in the vicinity of the black holes moves at the same speed-nearly as fast as light-around both Sgr A* and M87*. But where gas takes days to weeks to orbit the larger M87*, in the much smaller Sgr A* it completes an orbit in mere minutes,” he said. “This means the brightness and pattern of the gas around Sgr A* was changing rapidly as the EHT Collaboration was observing it-a bit like trying to take a clear picture of a puppy quickly chasing its tail.”
The researchers had to develop sophisticated new tools that accounted for the gas movement around Sgr A*. While M87* was an easier, steadier target, with nearly all images looking the same, that was not the case for Sgr A*. The image of the Sgr A* black hole is an average of the different images the team extracted, finally revealing the giant lurking at the center of our galaxy for the first time.
The effort was made possible through the ingenuity of more than 300 researchers from 80 institutes around the world that together make up the EHT Collaboration. In addition to developing complex tools to overcome the challenges of imaging Sgr A*, the team worked rigorously for five years, using supercomputers to combine and analyze their data, all while compiling an unprecedented library of simulated black holes to compare with the observations.
Scientists are particularly excited to finally have images of two black holes of very different sizes, which offers the opportunity to understand how they compare and contrast. They have also begun to use the new data to test theories and models of how gas behaves around supermassive black holes. This process is not yet fully understood but is thought to play a key role in shaping the formation and evolution of galaxies.
“Now we can study the differences between these two supermassive black holes to gain valuable new clues about how this important process works,” said EHT scientist Keiichi Asada from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei. “We have images for two black holes-one at the large end and one at the small end of supermassive black holes in the universe-so we can go a lot further in testing how gravity behaves in these extreme environments than ever before.”
Progress on the EHT continues: a major observation campaign in March 2022 included more telescopes than ever before. The ongoing expansion of the EHT network and significant technological upgrades will allow scientists to share even more impressive images as well as movies of black holes in the near future.
The EHT consortium consists of 13 stakeholder institutes; the Academia Sinica Institute of Astronomy and Astrophysics, University of Arizona, Center for Astrophysics | Harvard & Smithsonian, University of Chicago, East Asian Observatory, Goethe-Universitaet Frankfurt, Institut de Radioastronomie Millimétrique, Large Millimeter Telescope, Max Planck Institute for Radio Astronomy, MIT Haystack Observatory, National Astronomical Observatory of Japan, Perimeter Institute for Theoretical Physics and Radboud University.