An image that captures streaks of polarised light swirling around a supermassive black hole is providing new insight into how galaxies can project streams of energy thousands of light-years outward from their core.
Black holes are places where the pull of gravity is so strong that even light cannot escape. Most surrounding matter gets sucked in, but some particles escape just moments before they are captured and are blown far out into space.
These bright jets of energy and matter are one of the galaxy’s most mysterious features. Researchers suspect the jets are launched and shaped by magnetic fields, but the evidence for this is limited.
The new observations, based on data collected by the Event Horizon Telescope (EHT) collaboration, which links data from eight radio telescopes in various continents to create an Earth-sized virtual telescope, could help to better understand this phenomenon.
“We are now seeing the next crucial piece of evidence to understand how magnetic fields behave around black holes,” said Monika Moscibrodzka, an assistant professor at Radboud University in the Netherlands and co-author of the research, which was published in the Astrophysical Journal Letters.
The EHT collaboration released the first image of a black hole in 2019, revealing a bright ring-like structure with a dark central region described as the black hole’s “shadow”. The black hole is located in a galaxy called Messier 87 (M87), 55m light-years away from Earth.
After capturing that image, the team discovered that a significant fraction of its surrounding light was polarised – meaning its waves are vibrating in one direction only. The light is thought to become polarised when it is emitted in hot regions of space that are magnetised.
Using the same data as for their first image, they have now analysed that polarised light and are using it to map the magnetic field lines at the black hole’s inner edge, and better understand how these fields act to keep hot gas out of the black hole.
Team member Dr Ziri Younsi at the University College London Mullard space science laboratory said: “These groundbreaking measurements of the polarisation of light produced at the edge of the black hole’s event horizon provide us with exciting new insights into the physical processes by which black holes feed on matter, and how they are able to power such prodigious relativistic outflows as astrophysical jets. In particular, they hint at the role played by magnetic fields in these processes.”
The team found that only theoretical models featuring strongly magnetised gas could explain what they were seeing at the event horizon – the boundary marking the limits of a black hole.
Co-author Dr Jason Dexter, at the University of Colorado in Boulder, US, said: “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.”
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