The Biggest Black Hole Jets Ever Seen Are 140 Milky Ways Long

Space is vast and filled with giants: asteroids that can put planets on new evolutionary trajectories, roiling stars, and enormous galaxies. Now, a team of researchers has found some of the hugest objects yet: gargantuan jets of material traveling at nearly the speed of light away from a black hole.

At 23 million light-years long, the black hole jets are the largest yet seen. How big is 23 million light-years, you ask? It’s the equivalent of 140 Milky Ways lined up end-to-end. The study describing the jet megastructure, dubbed Porphyrion after a mythological Greek giant, is published today in Nature.

“We present evidence that supermassive black holes do not only hold sway over their galaxies, but also the cosmic web,” said Martijn Oei, a researcher at the California Institute of Technology and lead author of the paper, in a press conference Monday. Conventional knowledge held that black hole jets stay “within or very close to their host galaxy,” Oei added, but Porphyrion makes abundantly clear that the jets attain the size of more major cosmic structures.

Porphyrion is an ancient jet structure, dating back to when the universe was just 6.3 billion years old (it is now nearly 14 billion years old). Oei added that if Porphyrion’s jets were as wide as our planet, the black hole that powers them would merely be the width of an amoeba. That’s the ridiculous scale of the extensive jets.

The jets were spotted in data from a sky survey made with Europe’s LOw Frequency ARray, or LOFAR. So far, over 10,000 similar faint jet megastructures have been identified in the LOFAR data, which covered about 15% of the sky. The team identified Porphyrion’s source galaxy by studying data from the Giant Metrewave Radio Telescope (GMRT) in India and the Dark Energy Spectroscopic Instrument (DESI) In Arizona. Finally, observations by the Keck Observatory in Hawai’i revealed the jet structure’s distance from Earth: about 7.5 billion light-years.

The exact conditions that yield a jet system like Porphyrion are unclear, but the research team found that the black hole that hosted it was expelling radiation into space as it pulled in nearby material with its immense gravitational force.

“In order to make a system like Porphyrion, we have to have a very strong accretion event, perhaps a merger event with another galaxy that’s bringing in a lot of gas to feed the black hole,” Martin Hardcastle, an astrophysicist at the University of Hertfordshire and co-author of the paper, said during the press conference.

“Astronomers believe that galaxies and their central black holes co-evolve, and one key aspect of this is that jets can spread huge amounts of energy that affect the growth of their host galaxies and other galaxies near them,” said co-author George Djorgovski, an astronomer at Caltech, in an institute release. “This discovery shows that their effects can extend much farther out than we thought.”

The same team discovered the previous record-holding jet system, named Alcyoneus, also after a mythological giant. That system was about 100 Milky Ways long. Besides magnetizing the universe, jets like Porphyrion and Alcyoneus spew so much energy that they could heat up the intergalactic medium by a million degrees locally, Oei said. What’s more, the jets may generate and spew magnetic fields that occupy the cosmic voids—vast expanses of empty space that sit between the tendrils of the cosmic web.

“We have observations that the universe is magnetized on many scales. Entire galaxies are magnetized, and also the filaments of the cosmic web and the empty voids between those filaments,” Oei said. “People are interested in this large-scale magnetism because it might tell us something about how the universe started out.”

“Any magnetic fields within them [the cosmic voids] may be a reflection of some process in the early universe, which could lead us to hints of new physics,” he added.

New instruments like the Square Kilometer Array will provide useful means of surveying the southerns sky (in their recent work, the team only scanned the northern sky). Additionally, automated methods of parsing images—computer vision, for example, or other AI techniques‚ may hasten the number of black hole jet megastructures the team can find.

Currently, the group knows of about 10,000, but that’s just “a small fraction of what you can find,” Oei said, speculating that there could be anywhere between 100,000 to 1 million such jet structures out there.