We are getting closer to solving the strange mystery of the hundreds of colossal threads hanging at the heart of the Milky Way.
For the first time, these long, magnetized filaments glowing in radio waves were observed emanating from other galaxies. Not only are they no longer unique to the Milky Way, the range of environments in which they can be found allows scientists to narrow down the mechanisms that create them.
Astrophysicist Farhad Yousefzadeh of Northwestern University in the United States first discovered the filaments of the Milky Way in the 1980s, and has been puzzling them ever since.
According to Yousefzadeh, there are two possible explanations. The first is an interaction between galactic winds and large clouds. The second is turbulence within weak magnetic fields driven by the motion of galaxies.
“We know a lot about the filaments in our galactic center, and now the filaments in the outer galaxies are starting to emerge as a new set of extragalactic filaments,” Yousefzadeh says.
“The underlying physical mechanisms of both sets of filaments are similar despite vastly different environments. The objects are part of the same family, but the filaments outside the Milky Way are older, more distant cousins — and I mean very Distant cousins (in time and space).
About 1,000 of the filaments, up to 150 light-years across and suspended in neat, oddly orderly arrangements like harp strings, have been detected in the Milky Way so far, most recently thanks to the MeerKAT radio telescope in South Africa.
Sensitive telescope observations of the galactic center – which penetrates the dense dust and gas that obscures much of its interior – has increased the number of previously known filaments tenfold. These radio observations also revealed that the filaments contain cosmic ray electrons that orbit magnetic fields at near the speed of light, and that magnetic fields are amplified along all of the filaments.
Without more information, figuring out why they are there, lounging quietly in the galactic center, would be difficult. The discovery of more filaments, in four different galactic groups between 163 million and 652 million light-years away, is a huge achievement.
“Having studied filaments in our galactic center all these years, I was very excited to see these very cool structures,” Yousefzadeh says. “Because we found these filaments elsewhere in the universe, it indicates that something global is going on.”
The newly discovered filaments outside the Milky Way differ from the filament-like structures in our galaxy in several very important ways. They are associated with jets and lobes of radio galaxies—massive structures that radiate out from the galactic center, and extend for vast distances on either side of the galactic plane. The filaments that extend from these jets and lobes are also much larger than the structures we see in the center of the Milky Way – between 100 and 1,000 times larger.
“Some of them are of astonishing length, up to 200 kiloparsecs,” Yousefzadeh says.
“This is four or five times larger than the size of our entire Milky Way galaxy. What is remarkable is that its electrons stay together on such a long scale. If an electron travels at the speed of light along the length of the filament, 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. Naturally, they extend into intergalactic space, often at right angles to planes. The Milky Way’s filaments appear to center around the galactic disk.
On the other hand, the similarities are strong. Galactic filaments and galactic filaments have the same length-to-width ratio, and the transmission mechanism of cosmic rays is the same. If the same mechanism is producing all threads, then it must be something that works at different scales.
Wind could be one such mechanism. Supermassive active black holes and rampant star formation can generate galactic winds that blast into intergalactic space. These winds can drive the faint clouds of gas and dust that drift through interstellar and intergalactic space, pushing matter together to create filamentous structures.
The simulation suggested another possibility: a perturbation of the medium caused by gravitational perturbations. Such turbulence can create vortices in the intergalactic medium, where weak magnetic fields get stuck, bent, and eventually stretched into filaments with 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 dramatically different phenomena create structures that look eerily similar.
“All of these extragalactic filaments are very old,” Yousefzadeh says.
“They’re almost from a different era in our universe and yet they point to the inhabitants of the Milky Way as having a common filament-forming origin. I think that’s pretty cool.”
Research published in Astrophysical Journal Letters.
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