Northwestern University astrophysicist Farhad Zadeh has been fascinated and intrigued by a family of large-scale, highly organized magnetic filaments hanging from the center of the Milky Way since he first discovered them in the early 1980s. .
Now, 40 years later, Zadeh remains just as fascinated, but perhaps a little less perplexed.
With a new discovery of similar filaments located in other galaxies, Zadeh and his collaborators have, for the first time, introduced two possible explanations for the filaments’ unknown origins. In a new article, published earlier this month in Letters from the Astrophysical JournalZadeh and his co-authors propose that the filaments could result from an interaction between large-scale wind and clouds or could come from turbulence inside a weak magnetic field.
“We know a lot about filaments in our own galactic center, and now filaments from outer galaxies are starting to appear as a new population of extragalactic filaments,” Zadeh said. “The underlying physical mechanisms of the two populations of filaments are similar despite very different environments. The objects are part of the same family, but filaments outside the Milky Way are older and more distant cousins - and I mean very distant (in time and space) cousins.”
An expert in radio astronomy, Zadeh is a professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences and a member of the Interdisciplinary Center for Astrophysical Exploration and Research (CIERA).
“Something universal is happening”
The first filaments Zadeh discovered stretched up to 150 light-years in length, rising near the Milky Way’s central supermassive black hole. Earlier this year, Zadeh added nearly 1,000 more filaments to his observation collection. In this batch, the one-dimensional filaments appear in pairs and clusters, often stacked equidistant, side by side like strings on a harp or spilling sideways like individual ripples in a waterfall.
Using radio telescope observations, Zadeh discovered that the mystifying filaments include cosmic ray electrons spinning along a magnetic field at near light speed. Although he puts together the puzzle of the composition of the filaments, Zadeh still wonders where they come from. When astronomers discovered a new population outside our own galaxy, it offered new opportunities to study physical processes in space surrounding the filaments.
The newly discovered filaments reside inside a galaxy cluster, a concentrated tangle of thousands of galaxies located a billion light-years from Earth. Some of the galaxies in the cluster are active radio galaxies, which appear to be breeding grounds for the formation of large-scale magnetic filaments. When Zadeh first saw these newly discovered filaments, he was amazed.
“After studying filaments in our own galactic center for all these years, I was extremely excited to see these extremely beautiful structures,” he said. “Because we found these filaments elsewhere in the universe, it hints that something universal is going on.”
Although the new population of filaments resembles that of our Milky Way, there are a few key differences. The filaments outside the Milky Way, for example, are much larger, between 100 and 10,000 times longer. They are also much older and their magnetic fields are weaker. Most of them hang curiously – at a 90-degree angle – from jets of a black hole in the vast nothingness of the intracluster medium, or the space wedged between galaxies within the cluster.
But the newly discovered population has the same length-to-width ratio as the Milky Way’s filaments. And both populations seem to transport energy by the same mechanisms. Closer to the jet, the electrons in the filaments are more energetic, but they lose energy as they travel farther down the filament. Although the jet from the black hole may provide the seed particles needed to create a filament, something unknown must be accelerating these particles to astonishing lengths.
“Some of them are incredibly long, up to 200 kiloparsecs,” Zadeh said. “It’s about four or five times larger than the size of our entire Milky Way. What’s remarkable is that their electrons stick together on such a long scale. If an electron traveled at the speed of light the along the filament, it would take 700,000 years. And they don’t travel at the speed of light.”
In the new paper, Zadeh and his collaborators hypothesize that the origins of the filaments could be a simple interaction between the galactic wind and an obstacle, such as a cloud. As the wind wraps around the obstacle, it creates a comet-like tail behind it.
“The wind comes from the motion of the galaxy itself as it rotates,” Zadeh explained. “It’s like when you stick your hand out the window of a moving car. There’s no wind outside, but you feel the air moving. When the galaxy moves, it creates wind that could push through places where cosmic ray particles are enough It sweeps through matter and creates a filamentary structure.”
Simulations, however, offer another viable possibility. When the researchers simulated an active and turbulent medium, long filamentous structures materialized. As radio galaxies move, Zadeh explained, gravity can affect the medium and stir it up. The medium then forms patches of swirling vortices. Once the weak magnetic field wraps around these vortices, it can be stretched, bent and amplified, eventually becoming elongated filaments with a strong magnetic field.
Although many questions remain, Zadeh is always amazed by new discoveries.
“All of these filaments outside of our galaxy are very old,” he said. “They almost belong to a different time in our universe and yet signal to the inhabitants of the Milky Way that there is a common origin for the formation of the filaments. I think that’s remarkable.”
F. Yusef-Zadeh et al, Populations of magnetized filaments in the intracluster medium and the galactic center, Letters from the Astrophysical Journal (2022). DOI: 10.3847/2041-8213/ac982a
Provided by Northwestern University
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