Astronomers image magnetic fields at the edge of M87's black hole
3/24/2021 9:01:00 AM
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 center of the M87 galaxy: 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.
University of Illinois at Urbana-Champaign Astronomy and Physics Professor Charles Gammie is part of the EHT theory and simulation working group. Gammie is the Willett Professor of Physics and a founding member of the Illinois Center for Advanced Studies of the Universe (ICASU).
Gammie says, “Our research group at Illinois developed numerical procedures that capture the turbulent flow of gas in the strong gravitational pull of the black hole and then simulate the production and propagation of polarized light, taking into account the black hole's gravitational field.
“Illinois graduate students George Wong and Ben Prather played a vital role in creating the simulations used to the analyze the new results. They were also part of the paper writing team for the polarization theory paper published today in The Astrophysical Journal.”
Monika Moscibrodzka, coordinator of the EHT Polarimetry Working Group and assistant professor at Radboud Universiteit in the Netherlands, is a former member of the Gammie research group at Illinois. She says, “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.”
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. They have discovered that a significant fraction of the light around the M87 black hole is polarized.
“This work is a major milestone: the polarization 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 Universitat de València, Spain. He adds that “unveiling this new polarized-light image required years of work due to the complex techniques involved in obtaining and analyzing the data.”
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.
“The newly published polarized 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 USA.
The bright jets of energy and matter that emerge from M87’s core and extend at least 5000 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 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. Researchers at the U of I created such models.
Gammie explains, “The simulations our group created allow us to ask whether a given magnetic field configuration in the gas around the black hole are consistent with the data: do they produce the right polarization pattern in simulated observations? The most successful models, it turns out, are ‘MAD’, which is an acronym for Magnetically Arrested Disk. In MAD models, the black hole pulls in as much magnetic field as it possibly can, until the field is just on the verge of breaking out and pushing back the inflowing gas.”
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. 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.
"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.
Observational publication: First M87 Event Horizon Telescope Results. VII. Polarization of the Ring, ApJL March 24, 2021 https://iopscience.iop.org/article/10.3847/2041-8213/abe71d
Theory publication: First M87 Event Horizon Telescope Results. VIII. Magnetic Field Structure near The Event Horizon, ApJL March 24, 2021 https://iopscience.iop.org/article/10.3847/2041-8213/abe4de