The Milky Way’s mysterious filaments have ‘older, distant cousins’


The Milky Way's mysterious filaments have 'older, distant cousins'

Extensive magnetic filaments are leaking down from the jet of the black hole, which resides in a distant cluster galaxy. Credit: Rudnick and the collaborators, 2022

Northwestern University astrophysicist Farhadzadeh has been fascinated and mystified by a family of large-scale, highly organized magnetic filaments hanging in the center of the Milky Way since he first spotted them in the early 1980s.

Now, 40 years later, Zadeh is still just as fascinated—but perhaps a little less bewildered.

With a new discovery of similar threads found in another galaxiesZadeh and his collaborators have, for the first time, offered two possible explanations for the unknown origins of the filaments. In a new research paper, published earlier this month in Astrophysical Journal LettersZadeh and colleagues suggest that the filaments may result from an interaction between winds and clouds on a large scale, or they may arise from disturbance within a weak magnetic field.

“We know a lot about the filaments in our galactic center, and now the filaments in the outer galaxies are starting to emerge as a new set of extragalactic filaments,” Zadeh said. “The underlying physical mechanisms of both sets of filaments are similar despite the very different environments. The objects are part of the same family, but the filaments outside the Milky Way are older, more distant cousins ​​— and I mean very distant (in time and space) cousins.”

Expert in radio astronomyZadeh is Professor of Physics and Astronomy at Northwestern’s Weinberg College of Arts and Sciences and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).

Something global is happening

The first hairs found by Zadeh extended to 150 light years Long, towering near the Milky Way’s central supermassive black hole. Earlier this year, Zadeh added nearly 1,000 more strings to his collection of notes. In this installment, the one-dimensional strings appear in pairs and groups, often evenly stacked, side by side like strings on a harp or spilling sideways like individual ripples in a waterfall.

using notes from radio telescopesZadeh discovered that the mysterious filaments consist of cosmic ray electrons spinning along a magnetic field at close to the speed of light. Even though he’s piecing together the puzzle of what the threads are made of, Zadeh still wonders where they came from. When astronomers discovered a new population outside our galaxy, it opened up new opportunities for exploring physical processes in the space around the filaments.

The newly discovered filaments are located within A galaxy clusterA concentrated tangle of thousands of galaxies located a billion light-years from Earth. Some of the galaxies within the cluster are radio-active galaxies, which appear to be fertile grounds for the formation of large-scale magnetic filaments. When Zadeh saw these newfound filaments for the first time, he was amazed.

“Having studied filaments in our galactic center all these years, I was really excited to see these absolutely amazing structures,” he said. “Because we found these filaments elsewhere in the universe, it indicates that something global is going on.”

The Milky Way's mysterious filaments have 'older, distant cousins'

Close-up radio images of the magnetic filaments. The leftmost filament is from an outer galaxy. At 100 parsecs long, it rises above the other three filaments of the Milky Way, which are 28 parsecs, 12 parsecs, and 6 parsecs in length. Credit: Rudnick and the collaborators, 2022

Galactic giants

Although the new group of filaments looks similar to those in our own Milky Way galaxy, there are some key differences. The filaments outside the Milky Way, for example, are much larger—between 100 and 10,000 times longer. They are also older, and their magnetic fields are weaker. Most of them hang curiously—at a 90-degree angle—from the jets of the black hole into the vast nothingness of the midbrain, or the intergalactic space within the cluster.

But the newly discovered population has the same length-to-width ratio as the Milky Way’s filaments. Both groups seem to transmit energy through the same mechanisms. Closer to the plane, the filament’s electrons are most energetic, but they lose energy as they travel farther down strings. Although the black hole’s flow may provide the seed particles needed to create the filaments, something unknown must be accelerating these particles along to amazing lengths.

“Some of them are of astonishing length, up to 200 kilofarcs,” Zadeh said. “This is four or five times larger than the size of our entire Milky Way galaxy. What is remarkable is that its electrons stay together on such a long scale. If an electron travels at the speed of light along the length of the filament, it would take 700,000 years and they don’t travel at the speed of light.”

promising prospects

In the new paper, Zadeh and his collaborators hypothesize that the origins of the filaments could be a simple interaction between a galactic wind and an obstruction, such as a cloud. As the wind wraps around the obstacle, it creates a comet-like tail behind it.

“The wind comes from the movement of the galaxy itself as it rotates,” Zadeh explained. “It’s like sticking your hand out the window of a moving car. There’s no wind outside, but you can feel the air moving. When the galaxy moves, it creates winds that can push through places where cosmic ray particles are somewhat loose. It sweeps up matter and creates filamentous structure.”

Simulations, however, present another viable possibility. When the researchers simulated a turbulent active medium, long filamentous structures were achieved. Zadeh explained that as radio galaxies move, gravity can affect the medium and set it in motion. Then the center forms patches of swirling eddies. After a weak magnetic field wraps around these vortices, they can stretch, fold and swell—eventually becoming elongated filaments with a strong magnetic field.

Although many questions remain, Zadeh continues to marvel at the new discoveries.

“All of these filaments outside our galaxy are very old,” he said. “They’re almost from a different era in our universe and yet they point to the inhabitants of the Milky Way as having a common filament-forming origin. I think that’s pretty cool.”

more information:
F. Yusef-Zadeh et al, A population of magnetized filaments in the medium within the Mesocluster and Galactic Center, Astrophysical Journal Letters (2022). DOI: 10.3847/2041-8213/ac982a

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