Sagittarius A*

Astronomers Unveil Strong Magnetic Fields Spiraling at the Edge of Milky Way’s Central Black Hole

March 27, 2024

A new image from the Event Horizon Telescope (EHT) collaboration, with significant contribution of the Max Planck Institute for Radio Astronomy in Bonn, has uncovered strong and organized magnetic fields spiraling from the edge of the supermassive black hole Sagittarius A* (Sgr A*). Seen in polarized light for the first time, this new view of the monster lurking at the heart of the Milky Way Galaxy has revealed a magnetic field structure strikingly similar to that of the black hole at the center of the M87 galaxy, suggesting that strong magnetic fields may be common to all black holes. This similarity also hints toward a hidden jet in Sgr A*.

The results were published today in The Astrophysical Journal Letters.

Fig. 1: The Event Horizon Telescope (EHT) collaboration, who produced the first ever image of our Milky Way black hole released in 2022, has captured a new view of the massive object at the center of our Galaxy: how it looks in polarized light. This is the first time that astronomers have been able to measure polarization, a signature of magnetic fields, this close to the edge of Sagittarius A*. This image shows the polarized view of the Milky Way black hole. The lines mark the orientation of polarization, which is related to the magnetic field around the shadow of the black hole. — **Fig. 1:** The Event Horizon Telescope (EHT) collaboration, who produced the first ever image of our Milky Way black hole released in 2022, has captured a new view of the massive object at the center of our Galaxy: how it looks in polarized light. This is the first time that astronomers have been able to measure polarization, a signature of magnetic fields, this close to the edge of Sagittarius A*. This image shows the polarized view of the Milky Way black hole. The lines mark the orientation of polarization, which is related to the magnetic field around the shadow of the black hole.

© EHT Collaboration

**Fig. 1:** The Event Horizon Telescope (EHT) collaboration, who produced the first ever image of our Milky Way black hole released in 2022, has captured a new view of the massive object at the center of our Galaxy: how it looks in polarized light. This is the first time that astronomers have been able to measure polarization, a signature of magnetic fields, this close to the edge of Sagittarius A*. This image shows the polarized view of the Milky Way black hole. The lines mark the orientation of polarization, which is related to the magnetic field around the shadow of the black hole.

© EHT Collaboration

Scientists unveiled the first image of Sgr A*— which is approximately 27,000 light-years away from Earth— in 2022, revealing that while the Milky Way’s supermassive black hole is more than a thousand times smaller and less massive than M87’s, it looks remarkably similar. 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* revealed that the magnetic fields around the black hole giant allowed it to launch powerful jets of material back into the surrounding environment. Building on this work, the new images have revealed that the same may be true for Sgr A*.

“What we’re seeing now is that there are strong, twisted, and organized magnetic fields near the black hole at the center of the Milky Way galaxy,” says Sara Issaoun from the Harvard Center for Astrophysics, co-lead of the project. “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.”

Light is an oscillating, or moving, electromagnetic wave that allows us to see objects. Sometimes, light oscillates in a preferred orientation, and we call it “polarized.” Although polarized light surrounds us, to human eyes it is indistinguishable from “normal” light. In the plasma around these black holes, particles whirling around magnetic field lines impart a polarization pattern perpendicular to the field. This allows astronomers to see in increasingly vivid detail what’s happening in black hole regions and map their magnetic field lines.

In addition to the overall intensity, the polarisation information in the light reveals much more about the astrophysics, the properties of the gas, and the mechanisms that occur as a black hole is fed. By imaging polarised light from hot, glowing gas near black holes, astronomers can directly infer the structure and strength of the magnetic fields that govern the flow of gas and matter that the black hole ingests and ejects.

"Visualising black holes in polarised light isn't as easy as putting on a pair of polarised sunglasses. This is especially true for Sgr A*, which exhibits dynamic changes at a rapid rate, defying stillness for imaging purposes. Navigating through these fluctuations was a considerable challenge, but in the end we prevailed," says Maciek Wielgus from MPIfR. Capturing images of the supermassive black hole requires state-of-the-art tools beyond those used for the more stable M87*. Given the elusive nature of Sgr A*, its variable nature complicates the construction of even an unpolarised image. As a result, the initial image was created by stitching together multiple snapshots to account for Sgr A*'s motion. Wielgus explains: "Certain models proved too chaotic and turbulent to decipher a polarised image, but nature proved less harsh and allowed us to succeed."

Complementary observations with the Global mm-VLBI Array (GMVA), observing at a wavelength of 3.5 mm (cf. 1.3 mm for the EHT), provided an important contribution to the work presented here. Eduardo Ros, also from MPIfR, says: "The first step towards lifting the veil on the Galactic Centre were our GMVA observations together with ALMA, presented in January 2019. We continue to construct, brick by brick, the building of our understanding of black hole physics around supermassive black holes."

Scientists are excited to have images of both supermassive black holes in polarized light because these images, and the data that come with them, provide new ways to compare and contrast black holes of different sizes and masses. As technology improves, the images are likely to reveal even more secrets of black holes and their similarities or differences.

Christian M. Fromm, from the University of Würzburg says: "The striking similarity between the magnetic field structure of M87* and that of Sgr A* is remarkable because it raises the possibility that, despite differences in mass, size and environment, the physical mechanisms governing the feeding and jet ejection of a black hole may be common to all supermassive black holes." He continues: "We can now use this result to improve theoretical models and simulations, leading to a better understanding of how matter is affected near a black hole's event horizon."

The EHT has made several observations since 2017, and is scheduled to observe Sgr A* again in April 2024. Each year the images improve as the EHT adds new telescopes, wider bandwidths and new observation frequencies. Planned upgrades over the next decade will allow high-fidelity movies of Sgr A*, potentially revealing a hidden jet and allowing astronomers to observe similar polarisation features in other black holes. Meanwhile, extending the EHT into space will provide sharper images of black holes than ever before.

To better understand the physical processes behind the jets in active galactic nuclei and the role of magnetic fields in the vicinity of the central, powering black holes, the European Research Council awarded the M2FINDERS project to J. Anton Zensus, Director at the MPIfR. He says: "The revelation of these magnetic fields opens a window into the innermost regions of Sgr A*, where the interplay of gravity, magnetism and spacetime curvature reaches its zenith. As we delve deeper into this cosmic enigma, we expect further breakthroughs that will illuminate the fundamental nature of black holes and their influence on galactic ecosystems. An important step in understanding the inner workings of these extreme cosmic objects has been the iconic image of the black hole silhouette in Messier 87 and in the Galactic Centre, complemented today by the image of polarised light in the latter." He concludes: "Our M2FINDERS project is also a major contributor to the EHT effort since 2022.”

Fig. 2: At left, the supermassive black hole at the center of the Milky Way Galaxy, Sagittarius A*, is seen in polarized light, the visible lines indicating the orientation of polarization, which is related to the magnetic field around the shadow of the black hole. At center, the polarized emission from the center of the Milky Way, as captured by the infrared observatory SOFIA. At back right, the Planck Collaboration mapped polarized emission from dust across the Milky Way. — **Fig. 2:** At left, the supermassive black hole at the center of the Milky Way Galaxy, Sagittarius A*, is seen in polarized light, the visible lines indicating the orientation of polarization, which is related to the magnetic field around the shadow of the black hole. At center, the polarized emission from the center of the Milky Way, as captured by the infrared observatory SOFIA. At back right, the Planck Collaboration mapped polarized emission from dust across the Milky Way.

© Left: EHT Collaboration, Center: NASA/SOFIA, NASA/HST/NICMOS, Back right: ESA/Planck Collaboration

**Fig. 2:** At left, the supermassive black hole at the center of the Milky Way Galaxy, Sagittarius A*, is seen in polarized light, the visible lines indicating the orientation of polarization, which is related to the magnetic field around the shadow of the black hole. At center, the polarized emission from the center of the Milky Way, as captured by the infrared observatory SOFIA. At back right, the Planck Collaboration mapped polarized emission from dust across the Milky Way.

© Left: EHT Collaboration, Center: NASA/SOFIA, NASA/HST/NICMOS, Back right: ESA/Planck Collaboration

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Additional Information:

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 Submillimeter 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. Data were processed at the correlator facilities at the MPI für Radioastronomie in Bonn, Germany, and MIT/Haystack Observatory in Massachusetts, USA.

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.

Sara Issaoun is a NASA Hubble Fellowship Program Einstein Fellow at the Center for Astrophysics | Harvard & Smithsonian in Cambridge/Massachusetts and co-lead of the project. MPIfR astronomer Maciek Wielgus, together with Michael Janßen, from the Radboud University (Nijmegen/The Netherlands) and also affiliated to MPIfR, led the painstaking calibration of the data presented here. Eduardo Ros from the MPIfR is a member of the EHT collaboration and European GMVA scheduler. Christian M. Fromm is an astronomer at the University of Würzburg and also affiliated to the MPIfR. He led several tasks in the theory group at the EHT that developed numerical simulations to understand the physics of the object. J. Anton Zensus is Director at the MPIfR and Founding Chair of the Event Horizon Telescope Collaboration.

Following people affiliated to the MPIfR have contributed to this work (in alphabetical order): Alef, W.; Azulay, R.; Bach, U.; Baczko, A.K.; Britzen, S.; Desvignes, G.; Dzib, S.A.; Eatough, R.P.; Fromm, C.M.; Janssen, M.; Karuppusamy, R.; Kim, D.J.; Kim, J.Y.; Kramer, J.A.; Kramer, M.; Krichbaum, T.P.; Lisakov, M.; Liu, J.; Liu, K.; Lobanov, A.P.; Lu, R.S.; MacDonald, N.R.; Marchili, N.; Menten, K.M.; Müller, C.; Noutsos, A.; Ortiz-León, G.; Paraschos, G.F.; Pötzl, F.M.; Ros, E.; Rottmann, H.; Roy, A.L.; Savolainen, T.; Shao, L.; Torne, P.; Traianou, E.; Wagner, J.; Wharton, R.; Wielgus, M.; Witzel, G.; Zensus, J.A.; Zhao, G.Y.

Additional institutions in Germany participating at this work are: Institut für Theoretische Physik und Astrophysik, Universität-Würzburg, Frankfurt Institute of Advanced studies in Frankfurt, Institut für Theoretische Physik of the Goethe-Universität Frankfurt.

International Contacts for this press release include:

Dr. Sara Issaoun, Center for Astrophysics | Harvard & Smithsonian, USA; e--mail: sara.issaoun@cfa.harvard.edu

Dr. Mariafelicia De Laurentis, EHT Deputy Project Scientist, Universität von Neapel Federico II, Italien; e-mail: mariafelicia.delaurentis@unina.it

Prof. Dr. Geoffrey Bower, EHT Project Scientist, Institute of Astronomy and Astrophysics, Academic Sinica, Taipei; e-mail: gbower@asiaa.sinica.edu.tw

Prof. Dr. Huib Jan van Langevelde, EHT Project Director, JIVE and University of Leiden, The Netherlands; e-mail: langevelde@jive.eu