Astronomers Image Magnetic Fields at the Edge of M87’s Black Hole

Published on 24 March 2021


The Event Horizon Telescope (EHT) collaboration, who produced the first ever image of a black hole, has revealed today a new view of the massive object at the centre of the M87 galaxy: how it looks in polarised light. This is the first time astronomers have been able to measure polarisation, a signature of magnetic fields, this close to the edge of a black hole. The observations are key to explaining how the M87 galaxy, located 55 million light-years away, is able to launch energetic jets from its core.

"We are now seeing the next crucial piece of evidence to understand how magnetic fields behave around black holes, and how activity in this very compact region of space can drive powerful jets that extend far beyond the galaxy," says Monika Mościbrodzka, Coordinator of the EHT Polarimetry Working Group and Assistant Professor at Radboud University in the Netherlands.

On 10 April 2019, scientists released the first ever image of a black hole, revealing a bright ring-like structure with a dark central region - the black hole's shadow. Since then, the EHT collaboration has delved deeper into the data on the supermassive object at the heart of the M87 galaxy collected in 2017. They have discovered that a significant fraction of the light around the M87 black hole is polarised.

"This work is a major milestone: the polarisation of light carries information that allows us to better understand the physics behind the image we saw in April 2019, which was not possible before," explains Iván Martí-Vidal, also Coordinator of the EHT Polarimetry Working Group and GenT Distinguished Researcher at the University of Valencia, Spain. He adds that "unveiling this new polarised-light image required years of work due to the complex techniques involved in obtaining and analysing the data."

EHT collaboration members at JIVE are involved in developing, comparing, and testing these techniques. This is part of the JIVE commitment to build and improve dedicated software to enable astronomers to process their observations, as well as providing technical support.

Light becomes polarised when it goes through certain filters, like the lenses of polarised sunglasses, or when it is emitted in hot regions of space that are magnetised. In the same way polarised sunglasses help us see better by reducing reflections and glare from bright surfaces, astronomers can sharpen their vision of the region around the black hole by looking at how the light originating from there is polarised. Specifically, polarisation allows astronomers to map the magnetic field lines present at the inner edge of the black hole.

"The newly published polarised images are key to understanding how the magnetic field allows the black hole to 'eat' matter and launch powerful jets," says EHT collaboration member Andrew Chael, a NASA Hubble Fellow at the Princeton Center for Theoretical Science and the Princeton Gravity Initiative in the US.

The bright jets of energy and matter that emerge from M87's core and extend at least 5000 light-years from its centre are one of the galaxy's most mysterious and energetic features. Most matter lying close to the edge of a black hole falls in. However, some of the surrounding particles escape moments before capture and are blown far out into space in the form of jets.

Astronomers have relied on different models of how matter behaves near the black hole to better understand this process. But they still don't know exactly how jets larger than the galaxy are launched from its central region, which is as small in size as the Solar System, nor how exactly matter falls into the black hole. With the new EHT image of the black hole and its shadow in polarised light, astronomers managed for the first time to look into the region just outside the black hole where this interplay between matter flowing in and being ejected out is happening.

The observations provide new information about the structure of the magnetic fields just outside the black hole. The team found that only theoretical models featuring strongly magnetised gas can explain what they are seeing at the event horizon.

"The observations suggest that the magnetic fields at the black hole's edge are strong enough to push back on the hot gas and help it resist gravity's pull. Only the gas that slips through the field can spiral inwards to the event horizon," explains Jason Dexter, Assistant Professor at the University of Colorado Boulder, US, and coordinator of the EHT Theory Working Group.

To observe the heart of the M87 galaxy, the collaboration linked eight telescopes around the world, to create a virtual Earth-sized telescope, the EHT. This is a similar technique to what JIVE exploits for operating the European VLBI Network (EVN), another global network of telescopes. The impressive resolution obtained with the EHT is equivalent to that needed to measure the length of a credit card on the surface of the Moon.

This allowed the team to directly observe the black hole shadow and the ring of light around it, with the new polarised-light image clearly showing that the ring is magnetised. The results are published today in two separate papers in The Astrophysical Journal Letters by the EHT collaboration. The research involved over 300 researchers from multiple organisations and universities worldwide.

"The EHT is making rapid advancements, with technological upgrades being done to the network and new observatories being added. We expect future EHT observations to reveal more accurately the magnetic field structure around the black hole and to tell us more about the physics of the hot gas in this region," concludes EHT collaboration member Jongho Park, an East Asian Core Observatories Association Fellow at the Academia Sinica, Institute of Astronomy and Astrophysics in Taipei.

More information

This research was presented in two papers published today in The Astrophysical Journal.
The EHT collaboration involves more than 300 researchers from Africa, Asia, Europe, 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 are: ALMA, APEX, the IRAM 30-meter Telescope, the IRAM NOEMA Observatory, the James Clerk Maxwell Telescope (JCMT), the Large Millimeter Telescope (LMT), the Submillimeter Array (SMA), the Submillimeter Telescope (SMT), the South Pole Telescope (SPT), the Kitt Peak Telescope, and the Greenland Telescope (GLT).

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.

Very Long Baseline Interferometry (VLBI) is an astronomical method by which multiple radio telescopes distributed across great distances observe the same region of sky simultaneously. Data from each telescope is sent to a central "correlator" to produce images with higher resolution than the most powerful optical telescopes.

The European VLBI Network (EVN; is an interferometric array of radio telescopes spread throughout Europe, Asia, South Africa and the Americas that conducts unique, high-resolution, radio astronomical observations of cosmic radio sources. Established in 1980, the EVN has grown into the most sensitive VLBI array in the world, including over 20 individual telescopes, among them some of the world's largest and most sensitive radio telescopes. The EVN is administered by the European Consortium for VLBI, which includes a total of 15 institutes, including the Joint Institute for VLBI ERIC

The Joint Institute for VLBI ERIC (JIVE; has as its primary mission to operate and develop the EVN data processor, a powerful supercomputer that combines the signals from radio telescopes located across the planet. Founded in 1993, JIVE is since 2015 a European Research Infrastructure Consortium (ERIC) with seven member countries: France, Italy, Latvia, the Netherlands, United Kingdom, Spain, and Sweden; additional support is received from partner institutes in China, Germany and South Africa. JIVE is hosted at the offices of the Netherlands Institute for Radio Astronomy (ASTRON) in the Netherlands.

Image caption

A view of the M87 supermassive black hole in polarised light

The Event Horizon Telescope (EHT) collaboration, who produced the first ever image of a black hole released in 2019, has today a new view of the massive object at the centre of the Messier 87 (M87) galaxy: how it looks in polarised light. This is the first time astronomers have been able to measure polarisation, a signature of magnetic fields, this close to the edge of a black hole. 

This image shows the polarised view of the black hole in M87. The lines mark the orientation of polarisation, which is related to the magnetic field around the shadow of the black hole.

Credit: EHT Collaboration

Original: TIFF [10 Mb] – JPEG [8.8 Mb] Additional materials

Additional materials 

  • Video: Zoom into Messier 87, including the polarized emission of the ring (47s) - MP4 [81 Mb], UHD AVI [3.3 Gb] 
    • Caption: Zooming-in to the heart of M87 to see a new view of its black holeThis zoom video starts with a view of ALMA, a telescope in which ESO is a partner and that is part of the Event Horizon Telescope, and zooms-in on the heart of M87, showing successively more detailed observations. At the end of the video, we see the first ever image of a black hole - first released in 2019 - followed by a new image released in 2021: how this supermassive object looks in polarised light. This is the first time astronomers have been able to measure polarisation, a signature of magnetic fields, this close to the edge of a black hole. Credit: © ESO/L. Calçada, Digitized Sky Survey 2, ESA/Hubble, RadioAstron, De Gasperin et al., Kim et al., EHT Collaboration. Music: Niklas Falcke
  • Video: The M 87 image as seen through a light polarizer - YouTube | MP4 [11.7 Mb]
    • Caption: The M 87 image as seen with a polarizerRepresentation of the effect of a polarizer and how the polarization vectors in the M 87 ring image are produced from the combination of different linearly polarized components. Credit: © Iván Martí, Universitat de València, and the EHT Collaboration
  • Video: What Is Polarization - YouTube | MP4 [19.1 Mb]
    • Short caption: What Is Polarization | Event Horizon TelescopeLight is an oscillating electromagnetic wave. If the waves have a preferred direction of oscillation, they are polarized. In space, moving hot gas, or ‘plasma', threaded by a magnetic field emits polarized light. The polarized light rays that manage to escape the pull of the black hole travel to a distant camera. The intensity of the light rays and their direction are what we observe with the Event Horizon Telescope. Credit: © EHT Collaboration and Fiks Film
  • Video: Magnetic fields and black hole images - YouTube | Vimeo | MP4 [173 Mb]
    • Short caption: How Magnetic Fields Affect Black Hole Images | Event Horizon TelescopeBlack holes are enveloped in plasma. This plasma has magnetic fields-areas where magnetism affects how matter moves-threaded throughout. As the magnetic field grows stronger, it changes shape and the polarized light we measure exhibits different patterns. Credit: © EHT Collaboration and Crazybridge Studios.
  • Image: Group picture of the Polarimetry Workshop held in Bonn in July 2019 - JPEG [7.8 Mb]
    • Short caption: Group picture of the workshop which triggered the imaging the magnetic fields at the Max-Planck-Institut für Radioastronomie in Bonn, Germany, on July 15-19, 2021. Credit: © E. Traianou/MPIfR
  • Animated image: Transition between the observed polarization image and a best-fit theory image - GIF [7.6 Mb]
    • Caption: Transition between the observed polarization image and a best-fit theory imageCredit: © Monika Mościbrodzka & Sara Issaoun, Radboud Universiteit Nijmegen, and the EHT Collaboration
  • Image: Composite visual of M 87 jet and ring in polarization - Portrait: PDF [11.3 MB] | TIFF[22.9 MB]; Landscape: PDF [8.6 Mb] | TIFF [19.4 MB] 
    • Caption: View of the M 87 supermassive black hole and jet in polarized lightThis composite image shows three views of the central region of the Messier 87 (M87) galaxy in polarised light, namely, from top to bottom, with the Chile-based Atacama Large Millimeter/submillimeter Array (ALMA), the National Radio Astronomy Observatory's Very Long Baseline Array (VLBA) in the US, and with the Earth-sized telescope synthesized by the Event Horizon Telescope. Credit: EHT Collaboration; ALMA (ESO/NAOJ/NRAO), Goddi et al.; VLBA (NRAO), Kravchenko et al.; J.C. Algaba, I. Martí-Vidal


  • Observational publication: First M87 Event Horizon Telescope Results. VII. Polarization of the Ring, The Astrophysical Journal Letters, Vol. 910, L12, 24 March 2021 doi:10.3847/2041-8213/abe71e 
  • Theory publication: First M87 Event Horizon Telescope Results. VIII. Magnetic Field Structure near The Event Horizon, The Astrophysical Journal Letters, Vol. 910, L13, 24 March 2021 doi:10.3847/2041-8213/abe4de
  • Related publication: Polarimetric properties of Event Horizon Telescope targets from ALMA, Goddi, Martí-Vidal, Messias, and the EHT Collaboration, The Astrophysical Journal Letters, Vol. 910, in press, 24 March 2021 doi:10.3847/2041-8213/abee6a



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