Origin of Light-Years-Long Strands at Milky Way’s Center a Mystery

 

Meerkat Galactic Center

 

Traveling near the speed of light: an unprecedented new radio-telescope telescope image of the Milky Way Galaxy’s turbulent center has revealed nearly 1,000 mysterious magnetic filaments shown below stretching up to 150 light years long. The one-dimensional strands  are found in pairs and clusters, often stacked equally spaced, side by side like strings on a harp. “These filaments have been a mystery for several decades, and this study shows the substantial impact that MeerKAT is having in our studies of the center of our galaxy, and of these objects in particular,” Craig Heinke, professor of physics at the University of Alberta, wrote in an email to The Daily Galaxy.  

mystery strand at milky Way's Center

 

The new MeerKAT image of the Galactic center region is shown above with the Galactic plane running horizontally across the image. Many new and previously-known radio features are evident, including supernova remnants, compact star-forming regions, and the large population of mysterious radio filaments. The broad feature running vertically through the image is the inner part of the (previously discovered) radio bubbles, spanning 1400 light-years across the center of the Galaxy. The chaotic region is centered around the 4 million solar mass supermassive black hole that lurks in the center of our Galaxy. Colors indicate bright radio emission, while fainter emission is shown in greyscale. I. (Heywood, SARAO).

Using observations at radio wavelengths, Northwestern University astrophysicist Farhad Zadeh discovered the highly organized, magnetic filaments in the early 1980s. The mystifying filaments, he found, comprise cosmic ray electrons gyrating the magnetic field at close to the speed of light. The magnetic fields are amplified along the filaments, a primary characteristic all the filaments share.

 

Mosaic Image of the Milky Way Center

A mosaic image of the center of the Milky Way, captured with radio waves is shown below. The magnetic filaments are large, vertical slashes throughout the image. “It’s like modern art,” Zadeh said. “These images are so beautiful and rich, and the mystery of it all makes it even more interesting.”

 

Milky Way Center

The new image shown above has exposed 10 times more filaments than previously discovered, enabling Zadeh — a professor of physics and astronomy and a member of the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA)–and his team to conduct statistical studies across a broad population of filaments for the first time. This information potentially could help them finally unravel the long-standing mystery.

“We have studied individual filaments for a long time with a myopic view,” said Zadeh, the paper’s lead author. “Now, we finally see the big picture—a panoramic view filled with an abundance of filaments. Just examining a few filaments makes it difficult to draw any real conclusion about what they are and where they came from. This is a watershed in furthering our understanding of these structures.”

Mystery Object at Milky Way Center –A Failed Star or Massive Unknown Planet?

To construct the image with unprecedented clarity and detail, astronomers spent three years surveying the sky and analyzing data at the South African Radio Astronomy Observatory (SARAO). Using 200 hours of time on SARAO’s MeerKAT telescope, researchers pieced together a mosaic of 20 separate observations of different sections of the sky toward the center of the Milky Way galaxy, 25,000 light years from Earth.

The full image will be published in an additional, accompanying paper—led by Oxford University astrophysicist Ian Heywood and co-authored by Zadeh—in a forthcoming issue of The Astrophysical Journal. Along with the filaments, the image captures radio emissions from numerous phenomena, including outbursting stars, stellar nurseries and new supernova remnants.

“I’ve spent a lot of time looking at this image in the process of working on it, and I never get tired of it,” Heywood said. “When I show this image to people who might be new to radio astronomy, or otherwise unfamiliar with it, I always try to emphasize that radio imaging hasn’t always been this way, and what a leap forward MeerKAT really is in terms of its capabilities. It’s been a true privilege to work over the years with colleagues from SARAO who built this fantastic telescope.”

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To view the filaments at a finer scale, Zadeh’s team used a technique to remove the background from the main image in order to isolate the filaments from the surrounding structures. The resulting picture astounded him.

Exploring the Strands Magnetic Fields

While many mysteries surrounding the filaments remain, Zadeh has been able to piece together more of the puzzle. In their latest paper, he and his collaborators specifically explored the filaments’ magnetic fields and the role of cosmic rays in illuminating the magnetic fields.

Linked to the Milky Way’s Supermassive Black Hole

The variation in radiation emitted from the filaments is very different from that of the newly uncovered supernova remnant, suggesting that the phenomena have different origins. It is more likely, the researchers found, that the filaments are related to past activity of the Milky Way’s central supermassive black hole rather than coordinated bursts of supernovae. The filaments also could be related to enormous, towering  balloon-like  radio-emitting bubbles, shown below, which Zadeh and collaborators discovered in 2019. 

The radio bubbles discovered in 2019 by MeerKAT shown below extend vertically above and below the plane of the galaxy in the image below. Many magnetized filaments can be seen running parallel to the bubbles. (Oxford, SARAO; Adapted from results published in Heywood et al. 2019.)

 

Meerkat Bubbles

 

When asked how the filaments could be related to the bubbles he discovered in 2019,  Zadeh replied: “It is just that spatial distributions in longitudes and latitudes of the bubble and the filaments are similar. I wish we could have more direct evidence.”

 

And, while Zadeh already knew the filaments are magnetized, now he can say magnetic fields are amplified along the filaments, a primary characteristic all the filaments share.

“This is the first time we have been able to study statistical characteristics of the filaments,” he said. “By studying the statistics, we can learn more about the properties of these unusual sources.

“If you were from another planet, for example, and you encountered one very tall person on Earth, you might assume all people are tall. But if you do statistics across a population of people, you can find the average height. That’s exactly what we’re doing. We can find the strength of magnetic fields, their lengths, their orientations and the spectrum of radiation.”

“The Hunt is On” — For Elusive Magnetic Fields from the Big Bang

Structured Appearance a Puzzle

Among the remaining mysteries, Zadeh is particularly puzzled by how structured the filaments appear. Filaments within clusters are separated from one another at perfectly equal distances—about the distance from Earth to the sun. “They almost resemble the regular spacing in solar loops,” he said. “We still don’t know why they come in clusters or understand how they separate, and we don’t know how these regular spacings happen. Every time we answer one question, multiple other questions arise.”

Do the Filaments Move or Change Over Time? 

Zadeh and his team also still don’t know whether the filaments move or change over time or what is causing the electrons to accelerate at such incredible speeds. “How do you accelerate electrons at close to the speed of light?” he asked. “One idea is there are some sources at the end of these filaments that are accelerating these particles.”

“The idea is that cosmic-ray wind can interact with a source where shocks accelerate particles and then create a tail behind the source,” replied Zadeh in an email to The Daily Galaxy about the sources at the end of these filaments that are accelerating these particles to near the speed of light. “This is analogous to cometary tails formed when the solar wind interacts with a comet. We are in the process of testing this picture with the VLA.” 

Zadeh and his team are currently identifying and cataloging each filament. The angle, curve, magnetic field, spectrum and intensity of each filament will be published in a future study. Understanding these properties will give the astrophysics community more clues into the filaments’ elusive nature.

“We’re certainly one step closer to a fuller understanding,” Zadeh said. “But science is a series of progress on different levels. We’re hoping to get to the bottom of it, but more observations and theoretical analyses are needed. A full understanding of complex objects takes time.”

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The Last Word –Origin a Mystery

“The filaments are immediately captivating, simply because when we look into space at any wavelength we very rarely see structures that are so long and thin. We have also only seen them in the extreme environment of the center of our Galaxy, and to see them now congregating there in huge numbers only further adds to their mystery,” wrote University of Oxford physicist and Senior Researcher in radio astronomy and co-author, Ian Heywood, in an email to The Daily Galaxy. 

“Our work from a couple of years back speculated that they may be connected to a powerful explosion in the Galactic center several million years ago,” Heywood notes in his email. “I personally like this idea because it’s a nice ‘clean’ origin story, but really there is still no consensus amongst astronomers as to the mechanism or mechanisms that produce these things. We hope that the unprecedented images that MeerKAT has made possible will play a role in cracking this four decade long puzzle.”

“The high-pass filtering technique described brings out even faint radio filaments, but also generates some noise from other objects that are not radio filaments,” Craig Heinke told The Daily Galaxy.” So I would hesitate to say we’ve seen 1000 filaments in the Milky Way’s center; up to 1000 filaments might be more technically accurate, follow-up work by the authors will certainly better characterize and enumerate them.” 

“The key result is probably the estimate of the magnetic field in these filaments, which implies that these filaments are regions where the Galaxy’s magnetic field is particularly enhanced, and likely accelerating particles to relativistic speeds,” wrote astrophysicist Heinke in an email to The Daily Galaxy. “However,” he notes, “the authors have shown that these filaments are unlikely, by themselves, to be capable of accelerating most of the relativistic particles seen in the Galactic Center, implying that most of these particles are accelerated by either supernovae or by accretion into the monster black hole Sgr A* at the Galactic Center. At this point, the origin of the filaments themselves is still quite uncertain.”

The study, “Statistical properties of the population of the galactic center filaments: The spectral index and equipartition magnetic field,” was supported by NASA and the National Science Foundation.

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Craig Heinke, Ian Heywood, Farhad Zadeh and Northwestern University 

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