One of the most mysterious and interesting known locations in our neighborhood of space is the center of our home galaxy, the Milky Way. It’s thought to contain a supermassive black hole, with a mass of some 4 million suns. Astronomers call this region and its possible black hole Sagittarius A* (aka Sgr A*, pronounced Sagittarius A-star). In 2016, Farhad Yusef-Zadeh of Northwestern University reported his discovery of an unusual filament in this region. The filament is about 2.3 light-years long and appears to curve around the site of the black hole. Now, another team of astronomers has employed a new technique to obtain a high-quality image of the curved filament. These astronomers said their new image supports the idea that the filament is pointing toward the black hole. The new image has led to some fascinating speculations as to the nature of this mystery filament.
With our improved image, we can now follow this filament much closer to the galaxy’s central black hole, and it is now close enough to indicate to us that it must originate there. However, we still have more work to do to find out what the true nature of this filament is.
Researchers have considered three possible explanations for the filament:
The first is that it is caused by high-speed particles kicked away from the supermassive black hole. A spinning black hole coupled with gas spiraling inwards can produce a rotating, vertical tower of magnetic field that approaches or even threads the event horizon, the point of no return for infalling matter. Within this tower, particles would be sped up and produce radio emission as they spiral around magnetic field lines and stream away from the black hole.
The second, more fantastic, possibility is that the filament is a cosmic string, theoretical, as-yet undetected objects that are long, extremely thin objects that carry mass and electric currents. Previously, theorists had predicted that cosmic strings, if they exist, would migrate to the centers of galaxies. If the string moves close enough to the central black hole it might be captured once a portion of the string crosses the event horizon.
The final option is that the position and the direction of the filament aligning with the black hole are merely coincidental superpositions, and there is no real association between the two. This would imply it is like dozens of other known filaments found farther away from the center of the galaxy. However, such a coincidence is quite unlikely to happen by chance.
Each of the scenarios being investigated would provide intriguing insight if proven true. The scientists’ statement continued:
For example, if the filament is caused by particles being ejected by Sgr A*, this would reveal important information about the magnetic field in this special environment, showing that it is smooth and orderly rather than chaotic.
The second option, the cosmic string, would provide the first evidence for a highly speculative idea with profound implications for understanding gravity, space-time and the Universe itself.
Evidence for the idea that particles are being magnetically kicked away from the black hole would come from observing that particles further away from Sgr A* are less energetic than those close in. A test for the cosmic string idea will capitalize on the prediction by theorists that the string should move at a high fraction of the speed of light. Follow-up observations with the VLA should be able to detect the corresponding shift in position of the filament.
Even if the filament is not physically tied to Sgr A*, the bend in the shape of this filament is still unusual. The bend coincides with, and could be caused by, a shock wave, akin to a sonic boom, where the blast wave from an exploded star is colliding with the powerful winds blowing away from massive stars surrounding the central black hole.
Co-author Miller Goss, from the National Radio Astronomy Observatory in Socorro, New Mexico, said:
We will keep hunting until we have a solid explanation for this object. And we are aiming to next produce even better, more revealing images.
Bottom line: Astronomers have obtained a new image of the 2.3-light-year-long filament that curves around Sagittarius A*, the region of our galaxy thought to contain a supermassive black hole.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.