We’re getting closer to solving the strange mystery thrown up by hundreds of giant filaments dangling through the heart of the Milky Way.
For the first time, these long, magnetized filaments, which glow in radio waves, have been observed emerging from other galaxies. Not only are they no longer unique to the Milky Way, the variety of environments in which they can be found allows scientists to narrow down the mechanisms that produce them.
Astrophysicist Farhad Yusuf-Zadeh of Northwestern University in the US first discovered the filaments of the Milky Way in the 1980s and has been puzzling over them ever since.
According to Yusuf-Zadeh, there are two possible explanations. The first is an interaction between galactic winds and large clouds; The second is turbulence in weak magnetic fields excited by the motion of galaxies.
“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,” says Yusuf-Zadeh.
“The underlying physical mechanisms for both populations of filaments are similar despite the very different environments. The objects are in the same family, but the filaments outside the Milky Way are older, more distant relatives – and I mean very distant (in time and space) cousins.”
Around 1,000 of the filaments, which are up to 150 light-years long and hang in oddly neat and orderly arrangements like harp strings, have been spotted in the Milky Way so far, most recently thanks to the MeerKAT radio telescope in South Africa.
The telescope’s sensitive observations of the galactic center — penetrating through the thick dust and gas that obscures much of what lies within — expanded the number of filaments known so far by a factor of ten. These radio observations also showed that the filaments contain cosmic-ray electrons spinning in magnetic fields at nearly the speed of light, and that magnetic fields are amplified along the entire length of all filaments.
Without more information, it would be difficult to figure out why they are there, just hanging around quietly in the galactic center. The discovery of additional filaments in four different galaxy clusters, between 163 million and 652 million light-years away, is a major breakthrough.
“After studying filaments in our own galactic center for all these years, I was very excited to see these incredibly beautiful structures,” says Yusuf-Zadeh. “Because we found these filaments elsewhere in the universe, it suggests something universal is happening.”
The newly discovered filaments outside the Milky Way differ in a few important respects from the threadlike structures of our galaxy. They are associated with jets and lobes of radio galaxies – giant structures erupting from the galactic center and stretching vast distances on either side of the galactic plane. The filaments emanating from these jets and lobes are also much larger than the structures seen at the center of the Milky Way – between 100 and 1,000 times larger.
“Some of them are astonishingly long, up to 200 kiloparsecs,” says Yusuf-Zadeh.
“That’s about four or 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 were to travel the length of the filament at the speed of light, it would take 700,000 years. And they don’t travel at the speed of light.”
They are also older and their magnetic fields are weaker. And of course they extend into intergalactic space, often at right angles to the jets. The filaments of the Milky Way appear to be centered on the galactic disk.
On the other hand, the similarities are strong. The galactic and extragalactic filaments have the same length-to-width ratio, and the transport mechanism of cosmic rays is the same. If the same mechanism produces all the filaments, it must be something that works on different scales.
Winds could be such a mechanism. Active supermassive black holes and rampant star formation can create galactic winds that rush into intergalactic space. These winds could penetrate the thin clouds of gas and dust drifting through interstellar and intergalactic space, pushing the material together to form filamentous structures.
Simulations suggested another possibility: turbulence in the medium created by gravitational perturbations. This turbulence can create vortices in the intergalactic medium around which weak magnetic fields become entangled, folded, and eventually expanded into filaments of strong magnetic fields.
It’s not a definitive answer—yet. We don’t even know for sure if the same mechanism is responsible for both types of filaments, or if very different phenomena produce structures that look eerily similar.
“All of these filaments outside of our galaxy are very old,” says Yusuf-Zadeh.
“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 research was published in The Letters of the Astrophysical Journal.
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