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

Two UMass Amherst researchers part of international team that captured the first-ever picture of a black hole
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
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

An international team of astronomers, including University of Massachusetts Amherst professors Gopal Narayanan and Peter Schloerb, has just revealed a new view of the massive black hole at the center of a galaxy located 55 million light-years away, known as the M87 galaxy. This new image, captured by the Event Horizon Telescope (EHT) collaboration, shows how M87 looks in polarized light, and is published today in two papers appearing in The Astrophysical Journal (links here and here). It 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 is able to launch energetic jets from its core.

“There is a super-massive black hole at the center of almost every galaxy,” Narayanan explains. These black holes power the galactic nuclei, which often launches high energy jets from the central parts of the galaxy. Understanding the physics connecting super-massive black holes and galactic jets has been difficult. That is where light polarization comes in.

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. “Magnetic fields and jets have previously been mapped,” says Narayanan, “but at lower resolution. Now we can zoom in to the energetic launching regions of these jets with incredible resolution and map the jets in a way that has never been done before.”

“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 Universiteit in the Netherlands.

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, 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 the interplay between matter flowing in and being ejected out happens. “What’s particularly notable about this observation,” says Schloerb, “is that by measuring polarization you’re really getting at the properties of the magnetic field, which is so important because it helps us understand the basic physics of how galaxies evolve.”

To observe the heart of the M87 galaxy, the collaboration linked eight telescopes around the world, including the Large Millimeter Telescope (LMT), jointly operated by UMass and the country of Mexico, and the largest of its kind. Narayanan, who leads UMass’s EHT team, built the radio-astronomical receiver for the LMT that helped to gather the data on the M87’s black hole. 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 setup 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 today 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.

“Unveiling this new polarized-light image required years of work due to the complex techniques involved in obtaining and analyzing the data,” says Iván Martí-Vidal, coordinator of the EHT Polarimetry Working Group and GenT Distinguished Researcher at the Universitat de València, Spain.

“The thing I’m most fascinated about is the follow-up,” says Narayanan. “We have even more sensitive receivers now, and with more sensitive instruments we can pull out more details in the magnetic fields. There’s even more exciting data and analysis ahead.”

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