Astronomers Image Magnetic Fields at the Edge of M87's Black Hole
A new image taken with the globe-spanning Event Horizon Telescope array reveals the black hole at the center of the M87 galaxy and its shadow in polarized light. For the first time, astronomers get to observe the interplay of inflowing and ejected matter just outside the black hole.

University Communications and Event Horizon Collaboration
March 24, 2021


The M87 black hole in polarized light
This image shows the polarized view of the black hole in M87. The lines mark the orientation of polarization, which is related to the magnetic field around the shadow of the black hole. EHT Collaboration

The Event Horizon Telescope collaboration, which produced the first-ever image of a black hole, has revealed a new view of the massive object at the center of the M87 galaxy, showing how it looks in polarized light. This is the first time astronomers have been able to measure polarization, 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.

To observe the heart of the M87 galaxy, the collaboration linked eight telescopes around the world, including the University of Arizona's ARO Submillimeter Telescope on Mt. Graham in Arizona, to create a virtual Earth-sized telescope, the Event Horizon Telescope, or EHT. The resolution obtained with the EHT is equivalent to that needed to measure the length of a credit card on the surface of the moon.

On April 10, 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. The researchers have discovered that a significant fraction of the light around the M87 black hole is polarized.

Light becomes polarized when it goes through certain filters, like the lenses of polarized sunglasses, or when it is emitted in hot regions of space that are magnetized. In the same way polarized 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 polarized.

Specifically, polarization allows astronomers to map the magnetic field lines present at the inner edge of the black hole. This allowed the team to directly observe the black hole shadow and the ring of light around it, with the new polarized-light image clearly showing that the ring is magnetized. The results are published in two separate papers in The Astrophysical Journal Letters by the EHT collaboration. The research involved over 300 researchers from multiple organizations and universities worldwide.


UArizona ARO Submillimeter Telescope on Mt. Graham
The UArizona ARO Submillimeter Telescope, one of eight among the Event Horizon Telescope Array and one of two in the array integrated by the University of Arizona, magnifies the evening sky as the sun sets on Mount Graham near Tucson, Arizona. Bob Demers/UANews

"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," said Monika Mościbrodzka, coordinator of the EHT Polarimetry Working Group and assistant professor at Radboud University in the Netherlands.

"The newly published polarized images are key to understanding how the magnetic field allows the black hole to 'eat' matter and launch powerful jets," said 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 5,000 light-years from its center 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 polarized 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 radio waves we observed come from hot electrons around strong magnetic fields near the M87 black hole," said Chi-kwan Chan, an assistant astronomer at UArizona's Steward Observatory. "This is the same physics that the Earth's magnetic fields protect us from the solar wind and cosmic rays."

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 magnetized 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," said Jason Dexter, assistant professor at the University of Colorado, Boulder, and coordinator of the EHT Theory Working Group.

"Though our 2020 observations were impacted by the pandemic, the addition of new stations in our April 2021 campaign, particularly the UArizona ARO 12-meter telescope on Kitt Peak, will give us the ability to more accurately and completely reconstruct the magnetic field structure around the black hole," said Dan Marrone, a professor at Steward Observatory who serves as EHT science council chair and co-principal investigator on the latest National Science Foundation award to support EHT operations in the U.S. "The next datasets we take should be the clearest by far."

The EHT collaboration involves more than 300 researchers from Africa, Asia, Europe, North America 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, the Large Millimeter Telescope, the Submillimeter Array, the UArizona ARO Submillimeter Telescope, the South Pole Telescope, the Kitt Peak Telescope and the Greenland Telescope.

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. 


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