Astronomers Unveil Strong Magnetic Fields Spiraling at the Edge of Milky Way's Central Black Hole

A new image from the Event Horizon Telescope (EHT) collaboration has uncovered strong and organised magnetic fields spiraling from the edge of the supermassive black hole Sagittarius A* (Sgr A*). Seen in polarised light for the first time, this new view of the monster lurking at the heart of the Milky Way galaxy has revealed a magnetic field structure strikingly similar to that of the black hole at the centre of the M87 galaxy, suggesting that strong magnetic fields may be common to all black holes. This similarity also hints toward a hidden jet in Sgr A*. The results were published today in The Astrophysical Journal Letters.

In 2022 scientists unveiled the first image of Sgr A* at press conferences around the world, including at the European Southern Observatory (ESO). While the Milky Way’s supermassive black hole, which is roughly 27 000 light-years away from Earth, is more than a thousand times smaller and less massive than M87’s, the first-ever black hole imaged, the observations revealed that the two look remarkably similar. This made scientists wonder whether the two shared common traits outside of their looks. To find out, the team decided to study Sgr A* in polarised light. Previous studies of light around the M87 black hole (M87*) revealed that the magnetic fields around it allowed the black hole to launch powerful jets of material back into the surrounding environment. Building on this work, the new images have revealed that the same may be true for Sgr A*.

“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.”

Light is an oscillating, or moving, electromagnetic wave that allows us to see objects. Sometimes, light oscillates in a preferred orientation, and we call it ‘polarised’. Although polarised light surrounds us, to human eyes it is indistinguishable from ‘normal’ light. In the plasma around these black holes, particles whirling around magnetic field lines impart a polarisation pattern perpendicular to the field. This allows astronomers to see in increasingly vivid detail what’s happening in black hole regions and map their magnetic field lines.

“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.”

But imaging black holes in polarised light isn’t as easy as putting on a pair of polarised sunglasses, and this is particularly true of Sgr A*, which is changing so fast that it doesn’t sit still for pictures. Imaging the supermassive black hole requires sophisticated tools above and beyond those previously used for capturing M87*, a much steadier target. EHT Project Scientist Geoffrey Bower from the Institute of Astronomy and Astrophysics, Academia Sinica, Taipei said, “Because Sgr A* moves around while we try to take its picture, it was difficult to construct even the unpolarised image,” adding that the first image was an average of multiple images owing to Sgr A*’s movement. “We were relieved that polarised imaging was even possible. Some models were far too scrambled and turbulent to construct a polarised image, but Nature was not so cruel.”

Mariafelicia De Laurentis, EHT Deputy Project Scientist and professor at the University of Naples Federico II, Italy, said, “With a sample of two black holes — with very different masses and very different host galaxies — it’s important to determine what they agree and disagree on. Since both are pointing us toward strong magnetic fields, it suggests that this may be a universal and perhaps fundamental feature of these kinds of systems. One of the similarities between these two black holes might be a jet, but while we’ve imaged a very obvious one in M87*, we’ve yet to find one in Sgr A*.”

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), in which ESO is a partner, and the ESO-hosted Atacama Pathfinder Experiment (APEX), both in northern Chile, were part of the network that made the observations, conducted in 2017.  

"As the largest and most powerful of the telescopes in the EHT, ALMA played a key role in making this image possible,” says ESO’s María Díaz Trigo, European ALMA Programme Scientist. “ALMA is now planning an ‘extreme makeover’, the Wideband Sensitivity Upgrade, which will make ALMA even more sensitive and keep it a fundamental player in future EHT observations of Sgr A* and other black holes."

The EHT has conducted several observations since 2017 and is scheduled to observe Sgr A* again in April 2024. Each year, the images improve as the EHT incorporates new telescopes, larger bandwidth, and new observing frequencies. Planned expansions for the next decade will enable high-fidelity movies of Sgr A*, may reveal a hidden jet, and could allow astronomers to observe similar polarisation features in other black holes. Meanwhile, extending the EHT into space would provide sharper images of black holes than ever before.