The universe is a stupendously vast and puzzling place. Millennia of scientific advances have incrementally increased our understanding of it, but every now and then, scientists still spy something shrouded in almost inexplicable mystery. Now such a puzzle has come in the form of filaments of electromagnetic radiation hundreds of thousands of light-years long—the likes of which have never before been seen.
Using the MeerKAT radio telescope in South Africa, astronomers were taking a peek at a bright galaxy near to the center of Norma, a merging galaxy cluster 230 million light-years from Earth. Called ESO 137-006, this particular galaxy shines extremely brightly in radio waves: A supermassive black hole at its heart shoots out twin jets of plasma that reach far beyond the edges of the galaxy’s rim. The charged particles in these jets, moving close to the speed of light, spin around magnetic fields and emit radio waves as they go. Such activity is spectacular but not at all out of the ordinary on cosmic scales.
MeerKAT’s keen radio eyes revealed an unexpected feature, however: titanic threads of radio emissions emerging from the maelstrom. Click here to view the spectacular image. These near-parallel strands seemed to be connecting the swirling lobes at the ends of the galaxy’s two plasma jets to each other. Each thread is gigantic, with the longest one measuring a stunning 261,000 light-years.
Threads like these have been seen elsewhere, including within the center of our own galaxy. But the Milky Way’s own threads—themselves not fully understood—are many thousands of times smaller than the gargantuan ones emerging from ESO 137-006, says Minh Huynh, an astronomer at the International Center for Radio Astronomy Research, who was not involved with the work.
Heino Falcke, a radio astronomer at Radboud University in the Netherlands, who was also not part of the research, has studied black hole jets his entire scientific career. But these filaments left him stumped. “This is very weird,” he says, comparing the relatively thin, lengthy tendrils to the legs of a spider. “It’s really hard to imagine how this could be made.”
The international team of astronomers behind the discovery, which was reported earlier this month in the journal Astronomy & Astrophysics, are at a loss to explain the filaments, too. Yet if anyone can do so, the Byzantine astrophysics of supermassive black holes and hyperactive galaxies—from their evolution over time to the ways they unleash their pent-up energy—should become a little more comprehensible.
Mpati Ramatsoku, a radio astronomer at Rhodes University in South Africa and lead author of the new study, was initially interested in examining a calmer galaxy near ESO 137-006. But MeerKAT was first trained on the latter: the idea was that if ESO 137-006’s staggering radio luminosity could help teach the telescope’s software to filter out its fireworks, less ostentatious astronomical objects in the peripheries could be better seen. “If you’re looking at something that’s small and weak and delicate, the big, booming source always gets in the way,” says study co-author Oleg Smirnov, head of the Radio Astronomy Research group at the South African Radio Astronomy Observatory.
As MeerKAT perused the showstopping radio galaxy, the cryptic filaments bridging its two jets came to light. “We spent quite a lot of time doubting these structures,” Ramatsoku says. They were initially presumed to be some sort of artifact that arose during image processing. But painstaking examinations of the data have nixed that possibility.
“I would eat my hat if it was an artifact,” Smirnov says. The team also wondered if the threads were massive structures in the foreground of the image that were unrelated to the galaxy. But they seemed to mirror the shapes of the jets and those jets’ billowing lobes. “If they’re in front, it’s one hell of a coincidence,” Smirnov adds.
If the threads arose from ESO 137-006’s shenanigans, they probably did not suddenly appear as if by magic. A general rule in radio astronomy is that a structure’s size is proportional to how long it took to be made, says Yvette Cendes, a radio astronomer at the Center for Astrophysics at Harvard University and the Smithsonian Institution, who was not involved with the study. These filaments stretch for hundreds of thousands of light-years, hinting at a lengthy manufacturing process.
The energy signature of the threads also suggests their constitution: just like the jets themselves, their radio emissions are likely coming from the spiraling of electrons in a magnetic field. “The ingredients are known,” Smirnov says. But the astronomical alchemy that forged these filaments in the first place is, for now, anyone’s guess.
ESO 137-006’s journey through the darkness provides a possible answer. It is falling toward the center of the Norma galaxy cluster at an almost unfathomable pace. A dense soup of ionized hydrogen, helium and other heavier elements exists in the space between galaxies. It is possible, Ramatsoku says, that this intergalactic broth is being dragged along for the ride, causing magnetic filaments concealed in the lobes of the ESO 137-006’s plasma jets to wash out.
The filaments could be relic emissions from older episodes of jet activity, left behind in the galaxy’s wake much like contrails from a soaring airplane. The turbulent environment of the galaxy cluster could have stirred them up, causing their electrons to reaccelerate and making these contrails light up again. But, Huynh says, these so-called radio phoenixes are expected to be far more diffuse as the contrails spread apart over time—yet ESO 137-006’s threads are roughly parallel and relatively thin.
MeerKAT is already trying to find similarly gargantuan filaments in other radio galaxies. It could be that these threads are unique to ESO 137-006. Alternatively, they could be hiding in galaxies throughout the universe. Both outcomes would be a revelation, Smirnov says. But until we know either way, it is likely that astronomers will remain in the dark. “There’s definitely a lot of extra physics to be learned,” Cendes says.