Powerful magnetic fields found near Milky Way's black hole

The Event Horizon Telescope (EHT) collaboration, featuring researchers from the Center for Astrophysics (CfA), has recently shared an image showing the supermassive black hole Sagittarius A* (Sgr A*) at the heart of the Milky Way Galaxy in polarized light. The image has revealed magnetic fields spiraling from the black hole’s vicinity. 

This discovery, which mirrors the magnetic field structure seen in the M87 galaxy’s central black hole, suggests that such powerful magnetic fields are a universal feature around black holes. 

The researchers unveiled the first image of Sgr A* – a black hole located about 27,000 light-years from Earth – in 2022, highlighting its striking resemblance to the far larger M87’s black hole, and thus sparking curiosity about their shared characteristics beyond mere appearance. 

Strong, twisted, and organized magnetic fields

To better understand these features, the experts studied Sgr A* through polarized light, a technique previously applied to M87* to clarify how the giant black hole’s magnetic fields could expel material into its surroundings. The new polarized light images of Sgr A* indicate a similar phenomenon might be at play around our galaxy’s core.

“What we’re seeing now is that there are strong, twisted, and organized magnetic fields near the black hole at the center of the Milky Way galaxy,” said co-lead author Sara Issaoun, a CfA NASA Hubble Fellowship Program Einstein Fellow, and an astrophysicist at the Smithsonian Astrophysical Observatory (SAO). 

“Along with Sgr A* having a strikingly similar polarization 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.”

Imaging polarized light

Light travels as an electromagnetic wave that oscillates, enabling us to perceive the world around us. At times, this oscillation occurs in a specific direction, resulting in what is known as “polarized” light. 

Even though both polarized and unpolarized light surround us, our human vision cannot differentiate between the two. Near black holes, however, the situation changes dramatically. Here, particles whirling around magnetic fields adopt a polarization orientation that aligns at right angles to these fields. 

Unique ability to peer into the vicinity of black holes

This polarization grants astronomers the unique ability to peer into the vicinity of black holes with enhanced clarity, allowing them to delineate the structure of magnetic fields and offering a clearer picture of the dynamic processes occurring within these enigmatic regions of space.

“By imaging polarized 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,” explained co-lead author Angelo Ricarte, a Harvard Black Hole Initiative Fellow. 

“Polarized light teaches us a lot more about the astrophysics, the properties of the gas, and mechanisms that take place as a black hole feeds.” 

Rapidly changing black holes

Capturing Sgr A* in polarized light was no small feat due to its rapid changes, which challenge imaging efforts. 

“It is exciting that we were able to make a polarized image of Sgr A* at all. The first image took months of extensive analysis to understand its dynamical nature and unveil its average structure,” said co-author Paul Tiede, a CfA postdoctoral fellow and SAO astrophysicist.

“Making a polarized image adds on the challenge of the dynamics of the magnetic fields around the black hole. Our models often predicted highly turbulent magnetic fields, making it extremely difficult to construct a polarized image. Fortunately, our black hole is much calmer, making the first image possible.”

Revealing the secrets of black holes

These polarized light images of both supermassive black holes provide invaluable data for contrasting black holes of different sizes and masses. As technology advances, these images are expected to reveal more about black holes’ secrets and their similarities or differences.

“M87* is much bigger, and it’s pulling in matter from its surroundings at a much faster rate. So, we might have expected that the magnetic fields also look very different. But in this case, they turned out to be quite similar, which may mean that this structure is common to all black holes,” said co-author Michi Bauböck, a postdoctoral researcher at the University of Illinois Urbana-Champaign. 

“A better understanding of the magnetic fields near black holes helps us answer several open questions – from how jets are formed and launched to what powers the bright flares we see in infrared and X-ray light.”

The Event Horizon Telescope

CfA is currently initiating ambitious projects to significantly advance the capabilities of the Event Horizon Telescope (EHT) within the coming decade. The forefront of these initiatives is the next-generation EHT (ngEHT) project, poised to revolutionize the current EHT framework. 

This upgrade aims to introduce additional radio dishes to the network, facilitate observations in multiple colors simultaneously, and enhance the sensitivity of the entire setup. 

The enhancements brought about by the ngEHT project are expected to empower the telescope array to capture real-time footage of supermassive black holes at the scale of their event horizons. 

Shedding new light on black holes

These dynamic visualizations will unveil intricate structures and phenomena occurring close to the event horizon, highlighting the intense gravitational forces at play as predicted by General Relativity and the processes involved in matter accretion and the launching of relativistic jets, which are instrumental in shaping cosmic structures on a grand scale.

Parallel to the ngEHT project, the Black Hole Explorer (BHEX) mission concept envisions extending the reach of the EHT beyond Earth’s confines into space, setting the stage for producing the most precise astronomical images ever achieved. BHEX is designed to capture and analyze the “photon ring,” a distinct ring-shaped feature that emerges from light strongly bent by the gravitational field of black holes. 

The dimensions and form of the photon ring encode vital information about a black hole’s mass and spin, offering insights into the growth patterns of black holes and their interactions with their surrounding galaxies. Together, these groundbreaking initiatives will likely shed new light on the surrounding black holes and elucidate their role in the cosmic tapestry.

The study is published in the Astrophysical Journal Letters.

Image Credit: EHT Collaboration

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