In January of this year, a team of astronomers from Northwestern University in Illinois made public a strange discovery: more than a thousand mysterious filaments, up to 150 light-years long, ‘dangling’ from the center of our galaxy. After long years of work, the researchers, led by Farhad Zadeh, were then able to take images of the intriguing phenomenon, and in them the filaments appeared in pairs, or in clusters, one next to the other like the strings of a harp. Something certainly disconcerting and difficult to explain.
Farhad Zadeh first discovered these strange structures in the 1980s, and was fascinated by them. Today, four decades later, his studies are beginning to shed light on the origin of the phenomenon.
Now, in a new study just published in The Astrophysical Journal Letters, the astronomer and his team announce that, for the first time, they have been able to observe similar filaments dangling from other galaxies. And by being able to compare with each other they have begun to draw conclusions.
“We know a lot about filaments from our own Galactic Center,” says Farhad Zadeh, “and now they are starting to appear in outer galaxies as well, as a new population of extragalactic filaments. The underlying physical mechanisms for both filament populations are similar despite the different environments. The objects are part of the same family, but the ones we observe outside the Milky Way are older distant cousins, and I mean very distant cousins in time and space.”
Using radio telescopes for his analysis, Zadeh discovered that the more than 1,000 filaments discovered in January dangling from the Milky Way contain electrons from cosmic rays spinning along a magnetic field at close to the speed of light. But although the puzzle of the composition of the filaments is not yet complete, the big remaining question is how they formed. Now, by discovering the same structures outside our own galaxy, new possibilities open up to solve the enigma.
MeerKAT radio telescope image of the central region of our galaxy, showing several filaments
The new filaments reside within a distant galaxy cluster, a concentrated tangle of thousands of individual galaxies located billions of light-years from Earth. Some of them are active radio galaxies, and they seem to be the perfect breeding ground for the formation of large-scale magnetic filaments. When Zadeh first saw them, he was amazed.
“After studying the filaments in our own Galactic Center for all these years -says the scientist- I was very excited to see these tremendously beautiful structures. The fact that we find these filaments in other parts of the Universe tells us that ‘something universal’ is going on.”
subtle differences
Although to the naked eye the new population of filaments resembles that of our Milky Way, there are notable differences between them. The filaments outside the Milky Way, for example, are much larger, between 100 and 10,000 times as long. They are also much older, and their magnetic fields are weaker. Most of them also ‘hang’ in a strange way, forming a 90 degree angle with the jets that the black hole emits into space.
Of course, there are also similarities. The newly discovered population, for example, has the same length-width ratio as the filaments of the Milky Way. And both populations seem to transport energy through the same mechanisms. Closer to the jet emitted by the black hole, the electrons have more energy, but lose it as they travel further down the filament. Although the black hole’s jet could provide the seed particles needed to create a filament, something unknown must be accelerating these particles to astonishing lengths.
“Some of them,” says Zadeh, “are astonishingly long, up to 200 kiloparsecs (652,313 light-years). That’s about four or five times larger than the size of our entire Milky Way. What’s remarkable is that its electrons stick together, even on such a long scale. If an electron traveled at the speed of light along the filament, it would take 700,000 years to go through it. And they don’t travel at the speed of light.”
Two viable hypotheses
In their new study, Zadeh and his collaborators put forward two possible hypotheses about the origin of the filaments. For one, they could be a simple interaction between the galactic wind and an obstacle, such as a cloud of gas and dust. As the wind wraps around the obstacle, it creates a comet-like tail behind it.
“The wind,” explains Zadeh, “comes from the movement of the galaxy itself as it rotates. It’s like when you stick your hand out the window of a moving car. There is no wind outside, but you feel the air moving. When the galaxy moves, it creates wind that could be pushing at places where the cosmic ray particles are quite spread out. It sweeps up the material and creates a filamentary structure.”
The simulations, however, suggest a second possibility. When the researchers simulated an active, turbulent medium, long filamentary structures appeared. As radio galaxies move, Zadeh explains, gravity can affect the medium and agitate it, so that spinning eddies are formed at some points. The galaxy’s weak magnetic field surrounds the eddies, and in doing so they can stretch, fold and amplify, thus becoming elongated filaments with a strong magnetic field.
Although there are still many doubts to be resolved, Zadeh still marvels at these strange structures.
“All these filaments outside our galaxy are very old,” he says. They are from a different era of our Universe, and yet they signal to the inhabitants of the Milky Way that there is a common origin for their formation. I think this is something remarkable.”