by Astronomers have obtained the first glimpse of the polarized light and magnetic fields around Sagittarius A* (Sgr A*), the supermassive black hole at the heart of the Millennium Way.
The historic observation made with the Event Horizon Telescope (EHT) has shown that the neatly ordered magnetic fields are similar to those surrounding the supermassive black hole at the heart of the galaxy M87. This is surprising since Sgr A* has a mass of about 4.3 million times that of the sun, but M87* is much more massive, with a mass equal to about 6.5 billion suns.
Therefore, a new EHT observation of Sarcel A* suggests that all black holes may have strong, well-organized magnetic fields in common. Also, because M87*’s magnetic fields drive powerful outflows or “jets,” the results suggest that Sgr A* may have a hidden and weak jet of its own.
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“This new image of the black hole at the center of our Milky Way, Sgr A*, tells us that there are strong, complex and ordered magnetic fields near the black hole,” Sara Issaoun, co-leader of research and NASA’s Hubble Fellowship Program. An Einstein Fellow at the Center for Astrophysics (CfA) at Harvard & Smithsonian told Space.com “For a while we believed that magnetic fields play a central role in how black holes work and eject matter in powerful jets.
“This new image, along with a similar polarization structure seen in the much larger and more powerful M87* black hole, shows that strong and ordered magnetic fields are critical to how black holes interact with the gas and the matter around them.”
Compare the magnetism of two supermassive black holes
The EHT is made up of many telescopes around the globe, including the Atacama Large Millimeter/Sub-millimeter Array (ALMA), which come together to create an Earth-sized telescope that is no stranger to making science history.
In 2017, the EHT captured the first image of a black hole and its surroundings, imaging M87* located about 53.5 million light-years from Earth. Two years after this image was revealed to the public in 2019, the EHT collaboration has once again revealed the first look at polarized light around a black hole, M87*.
Polarization occurs when the directional waves of light are focused at a specific angle. The magnetic fields generated by plasma whipping around black holes polarize the light at a 90 degree angle to themselves. That means scientists can “see” the magnetic fields around a black hole for the first time when the polarization around M87* was observed.
Later in 2022 it was revealed that the EHT had imaged a supermassive black hole much closer to Earth just 27,000 light years away, Sarcel A*, the black hole on which the Milky Way is sculpted.
Now, the EHT has finally provided scientists with an image of polarized light and, therefore, the magnetic fields around this supermassive black hole.
“Polarized light is what teaches us about magnetic fields, the properties of gas, and the mechanisms that occur as a black hole is fed,” said Issaoun. “Given the additional challenges of imaging Sgr A*, it’s honestly surprising that we were able to get a polarized image in the first place!”
These challenges arose despite Sgr A* being closer to Earth, as the smaller size of the Milky Way’s supermassive black hole means that matter colliding around it at close to light speeds is hard to imagine. M87* is much larger, which means that the material, traveling at more or less the same speed, takes much longer to complete a circuit, making it easier for the Capture EHT.
By overcoming these difficulties it is now possible to compare two black holes at opposite ends of the supermassive black hole spectrum, one with billions of the mass of the sun and another with a mass of millions of times that of our star. The initial conclusion is that these magnetic fields are very similar to each other.
“This similarity was surprising because M87* and Sgr A* are very different black holes,” Issaoun said. “M87* is a rather special black hole: it is 6 billion solar masses, resides in a giant elliptical galaxy, and emits a powerful jet of plasma visible at all wavelengths.
“Sgr A*, on the other hand, is extremely common: It’s 4 million solar masses, it lives in our typical Milky Way spiral galaxy, and it doesn’t seem to have a jet at all.”
Issaoun explained that the team hoped to learn about the different properties of the magnetic fields of M87* and Sgr A* by looking at the polarized part of the light.
“One might be more ordered and stronger, and the other more disordered and weak,” said Issaoun. “However, because they look similar again, it is now clear that these two different classes of black holes have very similar magnetic field geometries!”
The results suggest that a deeper investigation of Sgr A* may reveal previously undiscovered features.
Is the Milky Way’s supermassive black hole sending a hidden jet?
The polarization of light and the neat and strong magnetic fields of Score A*, and the fact that they closely resemble that of M87*, may indicate that our central black hole has been hidden from us until now.
“We expect strong and ordered magnetic fields to be directly linked to the launch of jets as we observed for M87*,” explained Issaoun. “Since Sgr A* appears to have a very similar geometry, with no observed jet, there may also be a jet lurking in Sgr A* waiting to be observed, which would be extremely exciting!”
Astronomers were not surprised not to see a jet from Marseille A*. That’s because M87* is surrounded by so much gas and dust that it consumes the equivalent of two to three suns every year. That means enough material to send its magnetic fields to its poles and explode out as jets.
On the other hand, Sgr A* consumes as little material as a person eating one grain of rice every million years. These observations suggest that our supermassive black hole may still have a jet on a diet; it’s just hard to see.
“There is a lot of evidence of possible outflows and even jets powered by the black hole in the past, but a jet in Sgr A* has never been imaged due to the difficult environment of the galactic center,” Issaoun said. a jet would be a major revelation about our black hole and a link to its history within our Milky Way.”
She added that the process that sends these jets is the most energetic mechanism in the entire universe, which greatly affects the heart of galaxies through, for example, the gas and dust needed to clean birth stars and impact to play with how galaxies grow and develop. That means discovering a jet that arose from Sgr A* would disrupt our understanding of the way the Milky Way changed to the shape astronomers observe today.
“It is so striking that such a small nucleus can cause large-scale damage in a galaxy, and it all starts at the edge of the central black hole, where these magnetic fields control,” continued Issaoun.
Issaoun said that with these two very different polarized images of black holes, scientists now have very strong evidence that these cosmic titans have strong magnetic fields everywhere.
“The next step,” she said, “is figuring out how that geometry connects to how these systems move, evolve and light.”
The EHT will begin its 2024 observing campaign in early April, and the collaboration hopes to obtain multi-colour views of familiar black holes such as M87* and Sgr A* by observing them in different frequencies of light.
“Over the next decade, the next-generation EHT effort aims to add more telescopes to fill our Earth-sized virtual mirror and observe much more often,” Issaoun said. “With these increases in the EHT, we will be able to make polarized films of black holes and observe directly the dynamics between the M87* black hole and its jet.”
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In addition, the CfA researcher said that the EHT could eventually provide space-based assistance in observing black holes and their dynamics. One proposed mission that could help with this is the Black Hole Explorer (BHEX) mission concept, which adds one space telescope to the Earth-based EHT array.
“However many black holes rotate, it is believed that their spin is directly linked to why magnetic fields appear near the black hole as they look and how they can send jets,” said Issaoun. “With BHEX, we could image the sharp photon ring signature of black holes. This photon ring encodes the properties of the spacetime around the black hole, including the spin of the black hole!”
The EHT team’s research was published on Wednesday (March 27) in the Astrophysical Journal Letters.
