The Milky Way's Mysterious Filaments Have 'Older, Distant Cousins'

The Milky Way’s Mysterious Filaments Have ‘Older, Distant Cousins’

Northwestern University astrophysicist Farhad Zadeh has been fascinated and puzzled by a family of large-scale, highly organized magnetic filaments dangling at the center of the Milky Way since he first discovered them in the early 1980s.

Now, 40 years later, Zadeh is still just as intrigued – but maybe a little less confused.

With a new discovery of similar filaments in other galaxies, Zadeh and his collaborators have introduced, for the first time, two possible explanations for the filaments’ unknown origin. In a new paper published in The Astrophysical Journal Letters earlier this month, Zadeh and his co-authors suggest the filaments could result from an interaction between large-scale winds and clouds or from turbulence in a weak magnetic field.

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

An expert in radio astronomy, Zadeh is Professor of Physics and Astronomy at the Weinberg College of Arts and Sciences in the Northwest and a member of the Center for Interdisciplinary Investigation and Research in Astrophysics (CIERA).

“Something Universal Happened”

The first filaments Zadeh discovered were up to 150 light-years long, jutting out near the Milky Way’s central supermassive black hole. Earlier this year, Zadeh added nearly 1,000 more filaments to his collection of observations. In this stack, the one-dimensional filaments appear in pairs and clusters, often stacked side by side at equal distances, side by side like strings on a harp, or spilling sideways like single waves in a waterfall.

Close-up radio images of the magnetic filaments. The leftmost filament is from an outer galaxy. At 100 kiloparsecs long, it towers over the three other filaments of the Milky Way, which are 28 parsecs, 12 parsecs, and 6 parsecs long.

Using observations from radio telescopes, Zadeh discovered that the mysterious filaments were composed of cosmic ray electrons orbiting along a magnetic field at nearly the speed of light. Despite piecing together the puzzle that made up the threads, Zadeh still wondered where they came from. When astronomers discovered a new population outside of our own galaxy, it provided new opportunities to study the physical processes in space around the filaments.

The newly discovered filaments are found in a galaxy cluster, a concentrated tangle of thousands of galaxies located a billion light-years from Earth. Some of the galaxies within the cluster are active radio galaxies that appear to be hotbeds for the formation of large-scale magnetic filaments. When Zadeh first saw these newly exposed filaments, he was amazed.

“Having studied filaments in our own galactic center for all these years, I was very excited to see these incredibly beautiful structures,” he said. “Because we found these filaments elsewhere in the universe, it suggests something universal is happening.”

galactic giants

Although the new population of filaments looks similar to those in our Milky Way, 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 much older and their magnetic fields are weaker. Most of them oddly hang – at a 90 degree angle – from black hole jets into the vast void of the intracluster medium, or the space sandwiched between galaxies within the cluster.

200 kiloparsecs

Length of one of the extragalactic filaments

But the newly discovered population has the same length-to-width ratio as the filaments of the Milky Way. And both populations appear to transport energy through the same mechanisms. Closer to the beam, the filaments’ electrons are more energetic, but they lose energy as they travel farther down the filament. Although the black hole’s jet could provide the seed particles needed to form a filament, something unknown must be accelerating those particles to amazing lengths.

“Some of them are astonishingly long, up to 200 kiloparsecs,” Zadeh said. “That’s about four to five times larger than the size of our entire Milky Way galaxy. What is remarkable is that their electrons stay together for so long. If an electron traveled along the filament at the speed of light, it would take 700,000 years. And they don’t travel at the speed of light.”

Promising possibilities

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

“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 is no wind outside, but you can feel the air moving. As the galaxy moves, it creates winds that could push through places where cosmic ray particles are fairly loose. It sweeps the material and creates a threadlike structure.”

“All of these filaments outside of our galaxy are very old. They almost come from a different epoch in our universe and yet they signal to the inhabitants of the Milky Way that there is a common origin for the formation of the filaments. I find that remarkable.” — Farhad Zadeh, astrophysicist

However, simulations offer another viable option. When researchers simulated an active, turbulent medium, long, threadlike structures materialized. When radio galaxies move, Zadeh explains, gravity can affect the medium and stir it up. The medium then forms patches of swirling vortices. After the weak magnetic field wraps around these vortices, it can be stretched, folded, and amplified—eventually becoming elongated filaments with a strong magnetic field.

Although there are still many unanswered questions, Zadeh is still amazed at the new discoveries.

“All of these filaments outside of our galaxy are very old,” he said. “They almost come from a different epoch in our universe and yet they signal to the inhabitants of the Milky Way that there is a common origin for the formation of the filaments. I find that remarkable.”

The study “Populations of magnetized filaments in the intracluster medium and the galactic center” was supported by NASA (award number 80GSFC21M0002).

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