PREVIEW

A spectacular new image from the Event Horizon Telescope (EHT) collaboration shows the magnetic fields surrounding the supermassive black hole at the heart of our Milky Way galaxy.
For the first time, these magnetic fields, seen in polarised light, have been observed spiraling from the edge of the black hole, Sagittarius A* (Sgr A*).
This discovery, published today in The Astrophysical Journal Letters, suggests a striking similarity between the magnetic field structure of Sgr A* and that of the black hole at the centre of the M87 galaxy, hinting at common traits among black holes.
“What we’re seeing now is that there are strong, twisted, and organised magnetic fields near the black hole at the centre of the Milky Way galaxy,” said Sara Issaoun, NASA Hubble Fellowship Program Einstein Fellow at the Center for Astrophysics | Harvard & Smithsonian, US, and co-lead of the project.
“Along with Sgr A* having a strikingly similar polarisation structure to that seen in the much larger and more powerful M87* black hole, we’ve learned that strong and ordered magnetic fields are critical to how black holes interact with the gas and matter around them.”
In 2022, scientists unveiled the first-ever image of Sgr A*, which is located roughly 27,000 light-years away from Earth. Despite being significantly smaller and less massive than the black hole in M87, initial observations revealed surprising similarities between the two. This prompted researchers to investigate further, leading to the study of Sgr A* in polarised light.
The latest images indicate the presence of strong, twisted, and organised magnetic fields near Sgr A*, resembling those observed around M87*. Sara Issaoun, co-lead of the project and NASA Hubble Fellowship Program Einstein Fellow, noted that this discovery underscores the critical role of magnetic fields in how black holes interact with their surrounding environment.
Polarised light, an oscillating electromagnetic wave, reveals intricate details about the structure and strength of magnetic fields surrounding black holes. By capturing polarised light emitted by hot gas near these black holes, astronomers can map out magnetic field lines and gain insights into the processes occurring within black hole regions.
“By imaging polarised light from hot glowing gas near black holes, we are directly inferring the structure and strength of the magnetic fields that thread the flow of gas and matter that the black hole feeds on and ejects,” said Harvard Black Hole Initiative Fellow and project co-lead Angelo Ricarte. “Polarised light teaches us a lot more about the astrophysics, the properties of the gas, and mechanisms that take place as a black hole feeds.”
However, imaging black holes in polarised light presents significant challenges, particularly with a target like Sgr A*, which is constantly in motion. Despite these obstacles, the EHT collaboration successfully captured polarised images of Sgr A*, thanks to the sophisticated tools and techniques employed.
To observe Sgr A*, the collaboration linked eight telescopes around the world to create a virtual Earth-sized telescope, the EHT. The Atacama Large Millimeter/submillimeter Array (ALMA), and the Atacama Pathfinder Experiment (APEX), both in northern Chile, were part of the network that made the observations, conducted in 2017.

This zoom video takes you to Sagittarius A*, the supermassive black hole at the centre of our galaxy, seen now for the first time in polarised light. The video begins at the Atacama Large Millimeter/submillimeter Array (ALMA), a telescope in which ESO is a partner and that is part of the Event Horizon Telescope (EHT). As we zoom into the heart of our galaxy, we switch from visible to infrared light to peer through the dense clouds of dust in this region. We see some stars orbiting very close to Sgr A*, observed with ESO’s Very Large Telescope Interferometer. Finally, we arrive at Sgr A*. The first image of this black hole was released in 2022. The swirling lines overlaid in this new image mark the orientation of polarisation, which is linked to the shape of the magnetic field around the black hole.
The various observations used here were taken at different times, by different teams and with different facilities, and put together for the purpose of the zoom effect. The images go from visible wavelengths at the beginning to infrared, with the very final image being taken at radio wavelengths.