Astronomers have captured the first image of magnetic fields and polarized light surrounding Sagittarius A* (Sgr A*), the supermassive black hole at the heart of Milly Way.
Historic observation with the Event Horizon Telescope (EHT) has revealed that regular magnetic fields bear similarities to the fields surrounding the supermassive black hole at the heart of the M87 galaxy. This is surprising, given that Sgr A* has a mass of about 4.3 million times the Sun, but M87* is much more monstrous, with a mass equivalent to about 6.5 billion Suns.
The new EHT observation of Sgr A* therefore suggests that strong and well-organized magnetic fields may be common to all black holes. Additionally, because M87*’s magnetic fields give rise to strong outbursts, or “jets,” the results suggest that Sgr A* may have a hidden, faint jet of its own.
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“This new image of the black hole at the center of the Milky Way, Sgr A*, tells us that there are strong, twisted and ordered magnetic fields near the black hole,” said study co-leader and NASA Hubble Fellow Sara Issaoun. “We have believed for some time that magnetic fields play a key role in how black holes feed and eject matter in powerful jets,” the Einstein Fellow at the Harvard & Smithsonian Center for Astrophysics (CfA) told Space.com.
“This new image, together with the strikingly similar polarization structure seen in the much larger and more powerful M87* black hole, shows that strong and regular magnetic fields are critical to how black holes interact with the gas and matter around them.”
Comparing the magnetism of two monster black holes
The EHT consists of several telescopes around the world, including the Atacama Large Millimeter/submillimeter Array (ALMA), which combine to form an Earth-sized telescope that is no stranger to making scientific history.
In 2017, EHT captured the first image of a black hole and its surroundings by imaging M87*, located approximately 53.5 million light-years from Earth. Two years after this image was made 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 orientation waves of light are directed at a certain angle. Magnetic fields created by plasma orbiting black holes polarize light at a 90-degree angle to them. This means that observing the polarization around M87* allows scientists to “see” magnetic fields around a black hole for the first time.
This was followed in 2022 by the revelation that the EHT had imaged a supermassive black hole much closer to Earth, just 27,000 light-years away, Sgr A*, the black hole around which the Milky Way is formed.
EHT eventually provided scientists with an image of polarized light and thus magnetic fields around this supermassive black hole.
“Polarized light is what gives us information about magnetic fields, the properties of gas, and the mechanisms that occur as the black hole feeds,” Issaoun said. “Given the additional challenges of imaging Sgr A*, it is surprising enough that we were able to obtain a polarization image in the first place!”
These difficulties arise despite Sgr A* being closer to Earth, because the smaller size of the Milky Way’s supermassive black hole means that material orbiting it at speeds close to the speed of light is difficult to image. M87* is much larger, which means that the material, more or less, takes much longer to complete a circuit when traveling at the same speed, making it easier for the EHT to capture.
Overcoming these challenges means that a comparison can now be made between two black holes at opposite ends of the supermassive black hole spectrum, one billions of times the mass of the Sun and the other millions of times the mass of our star. The first conclusion is that these magnetic fields are quite similar to each other.
“This similarity was particularly surprising because M87* and Sgr A* are very different black holes,” Issaoun said. said. “M87* is a very special black hole: 6 billion solar masses, it lives in a giant elliptical galaxy and ejects a powerful jet of plasma visible at all wavelengths.
“Sgr A*, on the other hand, is extremely common: it has a mass of 4 million solar masses, lives in our ordinary spiral Milky Way galaxy, and appears to have no jets.”
Issaoun explained that the team expects to learn the different properties of M87* and Sgr A*’s magnetic fields just by looking at the polarized part of the light.
“Maybe one would be more regular and strong, and the other would be more disordered and weak,” Issaoun added. “But because they still look similar, it is now quite clear that these two different classes of black holes have very similar magnetic field geometry!”
The results suggest that deeper investigation of Sgr A* may reveal hitherto undiscovered features.
Is the Milky Way’s supermassive black hole shooting out a hidden jet?
The polarization of Sgr A*’s light and uniform and strong magnetic fields and its close resemblance to M87* may indicate that our central black hole has been keeping a secret from us until now.
“We expect strong and regular magnetic fields, like what we observed for M87*, to be directly linked to the ejection of jets,” Issaoun said. “Since Sgr A*, which has no observed jet, appears to have a very similar geometry, perhaps Sgr A* also has a jet waiting to be observed, which would be very exciting!”
Astronomers were not surprised to see a jet from Sgr A*. This is because M87* is surrounded by so much gas and dust that it consumes the equivalent of two or three suns every year. That means plenty of material for their magnetic fields to direct toward the poles and explode into jets.
On the other hand, Sgr A* consumes so little material that it is equivalent to a person eating one grain of rice every million years. These observations suggest that our dieting supermassive black hole may still have a jet; It’s just hard to see.
“There has been plenty of evidence in the past of possible outflows and even black hole-powered jets, but a jet in Sgr A* has never been imaged due to the harsh environment of the galactic center,” Issaoun said. “The jet will be a major discovery about our black hole and a link to its history in the Milky Way.”
He added that the process that ejects these jets is the most energetic mechanism in the entire universe, significantly affecting the heart of galaxies, for example by clearing the gas and dust needed to give birth to stars, and influencing how galaxies grow and develop. This means that the discovery of a jet originating from Sgr A* will impact our understanding of how the Milky Way evolved to take on the shape astronomers observe today.
“It’s striking that such a small nucleus in a galaxy can cause such large-scale damage, and it all starts at the edge of the central black hole where these magnetic fields dominate,” Issaoun continued.
With these two polarized images of very different black holes, scientists now have very convincing evidence that strong magnetic fields are ubiquitous for these cosmic titans, Issaoun said.
“The next step,” he said, “involves finding out how this geometry relates to how these systems move, evolve and glow.”
EHT will launch its 2024 observation campaign in early April, with the collaboration aiming to obtain multicolored views of familiar black holes such as M87* and Sgr A* by observing them in different light frequencies.
“Over the next decade, the next-generation EHT effort aims to add more telescopes and make much more frequent observations to fill our virtual Earth-sized mirror,” Issaoun said. he added. “With these expansions of the EHT, we will be able to take polarized movies of black holes and directly observe the dynamics between the M87* black hole and its jet.”
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Additionally, the CfA researcher said the EHT could eventually receive space-based assistance to observe black holes and their dynamics. One proposed mission that could help with this is the Black Hole Explorer (BHEX) mission concept, which adds a single space telescope to the Earth-based EHT array.
“How much black holes spin is believed to be directly linked to their spin, how the magnetic fields near the black hole appear, and how they can eject jets,” Issaoun said. he concluded. “With BHEX, we were able to image the sharp photon ring signature of black holes. This photon ring encodes features of the space-time around the black hole, including the spin of the black hole!”
The EHT team’s research was published Wednesday, March 27, in Astrophysical Journal Letters.