Astronomers reveal first image of the black hole at the heart of the Milky Way
Breakthrough discovery: EHT´s unprecedented observations improve our understanding of what happens at the very centre of our galaxy
Astronomers have unveiled the first image of the supermassive black hole at the centre of our own Milky Way galaxy. This result provides overwhelming evidence that the object is indeed a black hole and yields valuable clues about the nature of such giants, which are thought to reside at the centers 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 Max Planck Institute for Radio Astronomy (MPIfR) in Bonn plays a major role in all the aspects of this discovery, from founding and establishing the EHT collaboration to the final production and interpretation of the data.
The results are published in a series of papers in a special issue of the Astrophysical Journal Letters.
The black hole image is a long-anticipated direct look at the massive object that resides at the very centre of our galaxy, known as Sagittarius A* (Sgr A*, pronounced "sadge-ay-star"). Scientists, awarded by the Nobel prize, had previously seen stars orbiting around something invisible, compact, and very massive at the centre of the Milky Way. Anton Zensus, director at the Max Planck Institute for Radio Astronomy (MPIfR) and EHT Board Founding Chairman, states: “Our discovery shows that this compact, massive object at the Galactic Centre is indeed a black hole. Today’s image provides the first direct visual evidence of it - for the first time we can take a look at the black hole in our own galaxy.”
Although we cannot see the black hole itself, because it is completely dark, glowing gas around it reveals a tell-tale 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 observed ring agrees with predictions from Einstein’s Theory of General Relativity”, says EHT Project Scientist Geoffrey Bower from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei. “These unprecedented observations have greatly improved our understanding of physical processes in galactic centers, and offer new insights on how these giant black holes interact with their surroundings.” The EHT team's results are being published today in a special issue of The Astrophysical Journal Letters.
Because Sgr A* 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 links together eight existing radio observatories across the planet to form a single “Earth-sized” virtual telescope. The EHT observed the black hole on multiple nights, collecting data for many hours in a row, similar to using a long exposure time on a camera. This virtual telescope is realised through the use of a high-performance computer, known as a correlator. The MPIfR correlator analysed half of the data of the 2017 observational campaign.
“It’s great that the APEX telescope could play such an important role in the development of the EHT and also participate in these actual observations of Sgr A*” says Karl Menten, a director at the MPIfR, who is the principal investigator of the APEX telescope. He adds: “Its contribution is essential for achieving a perfect calibration of the changing brightness of the source and for bringing the final evidence of the black hole shadow in our Galactic Centre.”
The breakthrough follows the EHT collaboration’s 2019 release of the first image of a black hole, called M87*, at the centre of the more distant galaxy Messier 87. 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,” says Sera Markoff, Co-Chair of the EHT Science Council and a professor of theoretical astrophysics at the University of Amsterdam, the 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, US, 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. 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, which helps to reveal the giant lurking at the centre of our galaxy for the first time. The detailed calibration of the data was possible thanks to the observation of the brightness changes by extracting the ALMA-only data from the observations, A team led by Maciek Wielgus (MPIfR) performed this task, presented in one of the four additional papers to the six major publications. One of the leaders of this detailed calibration analysis was Michael Janßen (MPIfR), one of the leading authors of the second paper in the Sgr A* publication series. Tests of general relativity and steps towards the proof of an event horizon were made possible by adding the results of other observations as compiled by Gunther Witzel (MPIfR), one of the leading authors of the sixth paper.
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 analyse their data, all while compiling an unprecedented library of simulated black holes to compare with the observations.
Michael Kramer, a director at MPIfR, and co-PI of the “Black Hole Cam” project, points out that while the M87 image was a major achievement, its use for testing gravity is limited. He explains: “For Messier 87 we didn’t have a reliable prior knowledge about the mass of the black hole. Here, it is very different. Thanks to measurements such as those by Reinhard Genzel we know both the distance and the mass very precisely, so we can compute the expected shadow size to compare it with the observations. And it fits well!” The “Black Hole Cam” project was financed by the European Research Council (ERC) and is a main stakeholder in the EHTC.
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,” says 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.”
“The results now presented are also based on many years of joint pioneering work between MPIfR and IRAM. They are ideally complementary to the results obtained at the Max Planck Institute for Extraterrestrial Physics in the infrared range with the groundbreaking GRAVITY+ instrument”, says Karl Schuster, director of the Institut de Radioastronomie Millimétrique (IRAM) in Grenoble, France. Progress on the EHT continues: a major observation campaign in March 2022 included more telescopes than ever before. Schuster adds: “Now, of course, we are all very excited to see what the EHT observations conducted in 2021 and 2022 with the participation of the powerful NOEMA observatory will reveal.”
The ongoing expansion of the EHT network and significant technological upgrades, such as new enhancements to the MPIfR correlator facility, new developments in recording systems and in a new generation of receivers being tested at the Effelsberg radio telescope will help to enable even more impressive images and movies of black holes in the near future.
The individual telescopes involved in the EHT in April 2017, when the observations were conducted, were: the Atacama Large Millimeter/submillimeter Array (ALMA), the Atacama Pathfinder Experiment (APEX), the IRAM 30-meter Telescope, the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope Alfonso Serrano (LMT), the Submillimeter Array (SMA), the UArizona Submillimeter Telescope (SMT), the South Pole Telescope (SPT). Since then, the EHT has added the Greenland Telescope (GLT), the NOrthern Extended Millimeter Array (NOEMA) and the UArizona 12-meter Telescope on Kitt Peak to its network.
ALMA is a partnership of the European Southern Observatory (ESO; Europe, representing its member states), the U.S. National Science Foundation (NSF), and the National Institutes of Natural Sciences (NINS) of Japan, together with the National Research Council (Canada), the Ministry of Science and Technology (MOST; Taiwan), Academia Sinica Institute of Astronomy and Astrophysics (ASIAA; Taiwan), and Korea Astronomy and Space Science Institute (KASI; Republic of Korea), in cooperation with the Republic of Chile. APEX, a collaboration between the Max Planck Institute for Radio Astronomy (Germany), the Onsala Space Observatory (Sweden) and ESO, is operated by ESO. The 30-meter Telescope is operated by IRAM (the IRAM Partner Organizations are MPG (Germany), CNRS (France) and IGN (Spain)). The JCMT is operated by the East Asian Observatory on behalf of the Center for Astronomical Mega-Science of the Chinese Academy of Sciences, the National Astronomical Observatory of Japan, ASIAA, KASI, the National Astronomical Research Institute of Thailand, and organizations in the United Kingdom and Canada. The LMT is operated by INAOE and UMass, the SMA is operated by Center for Astrophysics | Harvard & Smithsonian and ASIAA and the UArizona SMT is operated by the University of Arizona. The SPT is operated by the University of Chicago with specialized EHT instrumentation provided by the University of Arizona.
The Greenland Telescope (GLT) is operated by ASIAA and the Smithsonian Astrophysical Observatory (SAO). The GLT is part of the ALMA-Taiwan project, and is supported in part by the Academia Sinica (AS) and MOST. NOEMA is operated by IRAM and the Univ. Arizona 12-meter telescope at Kitt Peak is operated by the University of Arizona.
Black holes are the only objects we know of where mass scales with size. A black hole a thousand times smaller than another is also a thousand times less massive.
The co-authors of the ten publications affiliated to the Max Planck Institute for Radio Astronomy are following individuals, in order of appearance at the summary publication (Paper I): W. Alef, R. Azulay, U. Bach, A.K. Baczko, S. Britzen, G. Desvignes, S.A. Dzib, R.P. Eatough, C.M. Fromm, M. Janßen, R. Karuppusamy, D.J. Kim, J.Y. Kim, M. Kramer, T.P. Krichbaum, M. Lisakov, J. Liu, K. Liu, A.P. Lobanov, R.S. Lu, N. Marchili, K.M. Menten, C. Müller, A. Noutsos, G.N. Ortiz-León, G.F. Paraschos, F.M. Pötzl, E. Ros, H. Rottmann, A.L. Roy, T. Savolainen, L. Shao, P. Torne, E. Traianou, J. Wagner, N. Wex, R. Wharton, M. Wielgus, G. Witzel, J.A. Zensus, A. Bertarini, M. Ciechanowicz, S. Dornbusch, D.A. Graham, S. Heyminck, D. Muders, J.P. Pérez-Beaupuits, & G. Wieching.