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

Large-scale magnetic filaments spill downward from a black hole's jet, located in a distant cluster galaxy. Credit: Rudnick and collaborators, 2022

Since his first discovery of a family of large-scale, highly structured magnetic filaments hanging in the center of the Milky Way in the early 1980s, Northwestern University astronomer Farhad Zadeh has been both captivated and perplexed by them.

After 40 years, Zadeh is still as enthralled—though probably a little less perplexed.

Zadeh and his colleagues have offered two potential theories for the filaments' unidentified origins for the first time with the recent finding of filaments that are comparable to those in other galaxies. The filaments may be caused by an interaction between large-scale wind and clouds, according to a recent research by Zadeh and his co-authors that was published earlier this month in The Astrophysical Journal Letters, or they may be caused by turbulence within a weak magnetic field.

We have a lot of knowledge about the filaments in our own Galactic Center, and now a new population of extragalactic filaments is starting to emerge, according to Zadeh. "Despite the very diverse surroundings, the basic physical mechanics for both populations of filaments are comparable. The objects are related, but the filaments beyond the Milky Way are older, more distant—in terms of both time and space—very distant—cousins."

Professor of physics and astronomy at Northwestern University's Weinberg College of Arts and Sciences and a specialist in radio astronomy, Zadeh is a part of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA).

"Something global is taking place,"

The initial filaments that Zadeh found towered close to the Milky Way's center supermassive black hole and were as long as 150 light years. Zadeh increased his observational database by over 1,000 strands earlier this year. The one-dimensional filaments in that batch occur in groups and pairs, often evenly spaced, stacked side by side like harp strings or pouring sideways like individual ripples in a cascade.

Zadeh revealed the mysterious filaments are made up of cosmic ray electrons that are whirling along a magnetic field almost as quickly as light. Zadeh continued to wonder where the filaments originated from despite having solved the mystery of what the filaments were comprised of. As a result of the discovery of a brand-new population outside of our own galaxy, scientists now have more opportunity to study the physical processes taking place around the filaments.

One billion light-years away from Earth, amid a concentrated tangle of millions of galaxies called a galaxy cluster, are the newly found filaments. A few of the cluster's galaxies are active radio galaxies, which seem to be favorable environments for the development of large-scale magnetic filaments. Zadeh was astounded when he first saw these recently discovered filaments.

I was ecstatic to see these very beautiful structures after studying filaments in our own Galactic Center for all these years, he remarked. These filaments were discovered in other parts of the cosmos, which suggests that a universal event is taking place.

Close-up radio images of the magnetic filaments. The filament at the far left is from an outside galaxy. At 100 kiloparsecs in length, it towers over the three other filaments from the Milky Way galaxy, which measure 28 parsecs, 12 parsecs and 6 parsecs in length. Credit: Rudnick and collaborators, 2022

Galactic giants

The new population of filaments resembles the filaments in our Milky Way in appearance, but there are some significant distinctions. For instance, the filaments outside of the Milky Way are substantially longer—between 100 and 10,000 times longer. They also have lesser magnetic fields and are considerably older. The majority of them oddly dangle at a 90° angle from black hole jets into the immense vacuum of the intracluster medium, or the region tucked in between the cluster's galaxies.

However, the newly found population has the same length to width ratio as the filaments of the Milky Way. And it seems that the same methods are used by both groups to transmit energy. The electrons in the filaments are more energetic closer to the jet, but as they descend farther, they become less energetic. The seed particles required to form a filament may be provided by the black hole's jet, but some unidentified force must be driving these particles over astounding distances.

Some of them are very long—up to 200 kiloparsecs, according to Zadeh. "That is around four or five times the size of the Milky Way as a whole. Their electrons' exceptional long-term bonding is what makes them so unique. It would take a proton 700,000 years to cross the length of the filament at the speed of light. Additionally, they do not move at the speed of light."

promising potential

In the new research, Zadeh and his colleagues postulate that the filaments' origins may be as simple as the collision of galactic wind with a barrier, like a cloud. The wind forms a comet-like tail behind the obstruction as it curves around it.

Wind originates from the galaxy's own rotation, according to Zadeh. "It is comparable to sticking your hand out of a moving car's window. Even if there isn't any wind outside, the air is still flowing. The wind produced by the galaxy's motion may be pushing through areas where the cosmic ray particles are rather loose. The material is swept, and a filamentary structure is produced."

However, simulations provide a different real-world potential. Long, filamentary formations appeared as scientists recreated an active, turbulent medium. According to Zadeh, gravitation may alter and agitate the medium as radio galaxies pass across it. Then, areas of whirling eddies develop in the medium. The weak magnetic field may get stretched, folded, and amplified after wrapping around these eddies, finally developing into elongated filaments with a high magnetic field.

Zadeh continues to be in awe of the fresh findings despite the numerous unanswered questions.

The man stated, "All of these strands beyond our galaxy are incredibly ancient. "They almost seem to be from a different time in the history of the cosmos, yet they nonetheless let the residents of the Milky Way know that the filaments have a common ancestor. I find this to be amazing."

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