FRANKFURT. In 2022, scientists of the EHT unveiled the first image of Sgr A* – which is approximately 27,000 light-years away from Earth – revealing that the Milky Way's supermassive black hole looks remarkably similar to M87's, even though it is more than a thousand times smaller and less massive. This made scientists wonder whether the two shared common traits outside of their looks. To find out, the team decided to study Sgr A* in polarized light. Previous studies of light around M87* had shown that the magnetic fields around the gigantic black hole allowed it to launch powerful jets of material back into the surrounding environment. Building on this work, the new images revealed that the same may be true for Sgr A*.
Imaging black holes, especially Sgr A*, in polarized light is not easy, because the ionized gas, or plasma, in the vicinity of the black hole orbits it in only a few minutes. Because the particles of the plasma swirl around the magnetic field lines, the magnetic field structures change rapidly during the recording of the radio waves by the EHT. Sophisticated instruments and techniques were required to capture the image the supermassive black hole.
Professor Luciano Rezzolla, theoretical astrophysicist at Goethe University Frankfurt, explains: "Polarized radio waves are influenced by magnetic fields and by studying the degree of polarization of the observed light we can learn how the magnetic fields of the black hole are distributed. However, unlike a standard image, which needs only information on the intensity of the light, creating a polarization map as the one we have just published is considerably harder. Indeed, our polarized image of Sgr A* is the result of a careful comparison between the actual measurements and the hundreds of thousands of possible images we can produce via advanced supercomputer simulations. Similar to the first image of Sgr A*, these polarized images represent an average of all measurements."
Rezzolla's fellow Project Scientist Geoffrey Bower from the Institute of Astronomy and Astrophysics, Academia Sinica, Taiwan adds, "Making a polarized image is like opening the book after you have only seen the cover. Because Sgr A* moves around while we try to take its picture, it was difficult to construct even the unpolarized image," adding that the first image was an average of multiple images due to Sgr A*'s movement. "We were relieved that polarized imaging was even possible. Some models were far too scrambled and turbulent to construct a polarized image, but nature was not so cruel."
"By imaging polarized light from hot glowing gas near black holes, we are directly inferring the structure and strength of the magnetic fields that thread the flow of gas and matter that the black hole feeds on and ejects," said Harvard Black Hole Initiative Fellow and project co-lead Angelo Ricarte. "Polarized light teaches us a lot more about the astrophysics, the properties of the gas, and mechanisms that take place as a black hole feeds."
Sara Issaoun, NASA Hubble Fellowship Program Einstein Fellow at the Center for Astrophysics, Harvard & Smithsonian and co-lead of the project, says "Along with Sgr A* having a strikingly similar polarization structure to that seen in the much larger and more powerful M87* black hole, we've learned that strong and ordered magnetic fields are critical to how black holes interact with the gas and matter around them."
Mariafelicia De Laurentis, EHT Deputy Project Scientist and professor at the University of Naples Federico II, Italy, also emphasizes the significance of the similarity between the magnetic field structures of M87* and Sgr A*, suggesting universal processes governing black hole feeding and jet launching despite differences in their properties. This finding enhances theoretical models and simulations, refining our understanding of black hole dynamics near the event horizon.
The Event Horizon Telescope Collaboration
The EHT has conducted several observations since 2017 and is scheduled to observe Sgr A* again in April 2024. Each year, the images improve as the EHT incorporates new telescopes, larger bandwidth, and new observing frequencies. Planned expansions for the next decade will enable high-fidelity movies of Sgr A*, may reveal a hidden jet, and could allow astronomers to observe similar polarization features in other black holes. Meanwhile, extending the EHT into space will provide sharper images of black holes than ever before.
The EHT collaboration involves more than 300 researchers from Africa, Asia, Europe, and North and South America. The international collaboration is working to capture the most detailed black hole images ever obtained by creating a virtual Earth-sized telescope. Supported by considerable international investment, the EHT links existing telescopes using novel systems — creating a fundamentally new instrument with the highest angular resolving power that has yet been achieved.
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 Institut de Radioastronomie Millimetrique (IRAM) 30-meter Telescope, the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope Alfonso Serrano (LMT), the Submillimeter Array (SMA), the UArizona AROSubmillimeter Telescope (SMT), the South Pole Telescope (SPT). Since then, the EHT has added the Greenland Telescope (GLT), the IRAM NOrthern Extended Millimeter Array (NOEMA) and the UArizona 12-meter Telescope on Kitt Peak to its network.
The EHT consortium consists of 13 stakeholder institutes: the Academia Sinica Institute of Astronomy and Astrophysics, the University of Arizona, the University of Chicago, the 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, Radboud University and the Smithsonian Astrophysical Observatory.
