The center of our Galaxy has been intensely studied for many years, but it still harbors surprises for scientists. A snake-like structure lurking near our galaxy’s supermassive black hole is one of the more intriguing discoveries. “Part of the thrill of science is stumbling across a mystery that is not easy to solve,” said Jun-Hui Zhao of the Harvard-Smithsonian Center for Astrophysics,. “While we don’t have the answer yet, the path to finding it is fascinating.”
“We have made good progress by obtaining new VLA (Very Large Array) observations of the mysterious region,” Zhao wrote in an email to The Daily Galaxy, “and are analyzing the data to nail down the nature of the snake-like filament source. Meanwhile, based on the new data, we have found some fascinating results on a population of Galactic center compact radio sources (GCCRs). The GCCRs may mark the headstones in the stellar graveyard at the center of our Galaxy.”
Snake-like filament about 2.3 light years long
In 2016, Farhad Yusef-Zadeh of Northwestern University reported the discovery of an unusual filament near the center of the Milky Way Galaxy using the NSF’s Karl G. Jansky Very Large Array (VLA). The filament is about 2.3 light years long and curves around to point at the supermassive black hole, called Sagittarius A* (Sgr A*), located in the Galactic center.
Another team of astronomers has employed a pioneering technique to produce the highest-quality image yet obtained of this curved object.
We still have more work to do to find out the true nature of this 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,” said Mark Morris of the University of California, Los Angeles, who led the study. “However, we still have more work to do to find out what the true nature of this filament is.”
Three Possible Explanations
The researchers have considered three main 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.
“Assuming they do exist,” reports LIGO, “the network of cosmic (super)strings formed in the early Universe would have evolved as the Universe expanded: strings stretched, interacted, oscillated, disintegrated. Massive computer simulations can describe this evolution from formation up to today, but they are challenging because many physical effects need to be taken into account.
Currently no observational evidence
“There is currently no observational evidence of the existence of cosmic strings,’ LIGO adds, “and the CMB (cosmic microwave background radiation) evidence proves they are not abundant. One of the most promising ways to detect these elusive objects is to search for the gravitational-wave radiation they would produce. Gravitational-wave emission is the main mechanism for cosmic strings to dissipate energy. When a string in a cosmic string network crosses itself, a loop separates from the string. In spite of all our efforts, no evidence of a cosmic string signal has been found in LIGO/Virgo data. As often in experimental physics, a null result does not mean we didn’t learn anything.”
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. 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.
Could have profound implications for understanding gravity, space-time and the Universe itself
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.
“We will keep hunting until we have a solid explanation for this object,” said co-author Miller Goss, from the National Radio Astronomy Observatory in Socorro, New Mexico. “And we are aiming to next produce even better, more revealing images.”
The Last Word
“We’re still pursuing what this filament might be — all possible explanations except one seem very interesting,” wrote UCLA’s Morris. “The one explanation that is not so interesting is that one of the many radio filaments seen toward the central few hundred parsecs (500 light years) of the Galactic center just happens to be accidentally superimposed along the line of sight in just such a way that it appears as if it is linked to the central supermassive black hole of our Galaxy,” explains Morris in an email to The Daily Galaxy. “The probability of that is, however, seems quite small.
“The more interesting hypothesis,” suggests Morris “is that the energy generated by the accretion flow onto the black hole produces highly energetic particles [electrons] that stream out from the black hole and are trapped in the local magnetic field just as the solar wind is trapped by the Earth’s magnetic field to produce the van Allen radiation belts. As the relativistic electrons stream along the magnetic field lines, they emit radio radiation that “illuminates” the magnetic field lines [flux tubes] into which they are trapped, creating a linear filament of emission.
A Black-Hole Tail
“A more exotic possibility,” says Morris, “is that the filament is a cosmic string, a theorized entity that has not yet been observed in the cosmos. If they exist, cosmic strings that cross through the event horizon of a black hole can conceivably become trapped, creating an eternally wagging and radio-emitting “tail” to the black hole.
“The black hole itself is such a bright radio source that it tends to drown out the radio emission from the portions of the filament nearest the black hole, so although the filament gets very close to the black hole and points right at it, we can’t yet be sure that it is connected,” concludes Morris. “Our efforts now are aimed at improving our images to see whether we can follow the radio filament much closer to the black hole’s event horizon.”
Image credit: Harvard CfA