New images shed new light on mysterious strands of the Milky Way

Unidentified filaments hang inexplicably from the turbulent center of the Milky Way, but a new study may have unlocked some of their secrets.

One-dimensional strands are found in pairs or clusters, sometimes arranged in equidistant stacks like the strings of a harp or guitar, spanning 150 light-years in length. Astronomer Farhad Yusef-Zadeh of Northwestern University discovered these magnetic filaments in the 1980s and determined that they are made up of cosmic ray electrons spinning in a magnetic field at near the speed of light.

Now, some 40 years later, Yusef-Zadeh and a team of researchers have captured new images that reveal 10 times more filaments than previously discovered. For the first time, this allowed them to conduct statistical studies on the filaments and unravel their dark mystery.

A mosaic image of the center of the Milky Way, showing the spectral index of the filaments. (Farhad Yusef-Zadeh)
A mosaic radio image of the center of the Milky Way galaxy with the background removed to isolate magnetic filaments. The filaments are large vertical slashes across the image. (Farhad Yusef-Zadeh)

“We studied individual filaments for a long time with myopic vision,” said Yusef-Zadeh, co-author of the new study published in The Astrophysical Journal Letters. “Now we finally see the big picture – a panoramic view filled with an abundance of filaments. Just looking at a few filaments makes it difficult to draw any real conclusion about what they are and where they came from. It This is a turning point in the deepening of our understanding of these structures.

The team of astronomers analyzed three years of data from the South African Radio Astronomy Observatory (SARAO). Using SARAO’s MeerKAT radio telescope, they cobbled together 20 separate observations of different sections of the cosmos some 25,000 light-years from Earth at the center of the Milky Way galaxy. The full image will be published in an accompanying article led by Ian Heywood of the University of Oxford. The image also captured radio emissions from the remnants of supernovae, or exploding stars, and star birthplaces.

The researchers explored the magnetic fields around the filaments and examined how cosmic rays illuminate them. Because the radiation emitted by the filaments is distinct from that of a recently discovered supernova remnant, they believe the two phenomena have different origins.

The filaments are likely a testament to the distant past of the supermassive black hole at the heart of the Milky Way, rather than bursts of supernova energy. However, the filaments can also be linked to gigantic bubbles emitting radio waves, discovered by Yusef-Zadeh in 2019.

While it was already known that filaments are magnetized, Yusef-Zadeh said team members can now study the statistical characteristics of filaments. “We can find the strength to [the] magnetic fields, their lengths, orientations and the radiation spectrum,” he said.

A radio image of the center of the Milky Way with part of the MeerKAT telescope array in the foreground. The plane of the galaxy is marked by a series of bright features, star bursts and regions where new stars are being born and runs diagonally across the image from bottom right to top center. The black hole at the center of the Milky Way is hidden in the brightest of these extended regions. The radio bubbles extend between the two closest antennas to the upper right corner. Many magnetized filaments can be seen parallel to the bubbles. In this composite view, the sky to the left of the second closest antenna is the night sky visible to the naked eye, and the radio image to the right has been magnified to highlight its fine features. (SARAO/Oxford/NRAO)

Yusef-Zadeh is still mystified by the way the filaments are structured. Within the clusters, the filaments are separated from each other at perfectly equal distances, approximately the distance from the Earth to the Sun.

“They almost look like the evenly spaced solar loops,” he said. Solar loops, or coronals, are magnetic loops beginning and ending on the Sun’s surface that project into the solar corona or atmosphere. The glowing ionized gas or plasma trapped in the loops makes them visible.

“We still don’t know why they cluster or understand how they separate, and we don’t know how these regular spacings occur. Every time we answer a question, several other questions arise,” Yusef-Zadeh said.

Adding to the mystery, astronomers don’t know if the filaments are changing or moving or what is causing their electrons to accelerate to near the speed of light. Yusef-Zadeh theorizes that there may be “sources at the end of these filaments that accelerate these particles.” He will then catalog each filament and later publish their angles, curves, magnetic fields, spectra and intensities to provide more clues to their elusive nature.

Edited by Siân Speakman and Kristen Butler

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