Astronomers have discovered the largest duo of black hole jets ever recorded, reaching a staggering length of 23 million light-years combined. This distance is comparable to the alignment of 140 Milky Way galaxies placed end to end.
Martijn Oei, a postdoctoral researcher at Caltech and the principal author of a new study published in Nature, states, “We are not talking about jets the size of our solar system or even the Milky Way; these jets span a total distance equal to that of 140 Milky Way diameters. The Milky Way would resemble a tiny dot within these colossal eruptions.”
This jet megastructure, named Porphyrion after a figure from Greek mythology, originates from a period when the universe was 6.3 billion years old, which is less than half of its current age of 13.8 billion years. These powerful outflows, with an energy output equal to trillions of suns, extend from above and below a supermassive black hole at the center of a far-off galaxy.
Before the discovery of Porphyrion, the largest confirmed jet system was Alcyoneus, which is also named after a giant from Greek mythology. Discovered in 2022 by the same team that identified Porphyrion, Alcyoneus stretches across a distance equal to about 100 Milky Ways. In comparison, the well-known Centaurus A jets, the nearest major jet system to Earth, measure about 10 Milky Ways.
The latest discovery indicates that these massive jet systems might have played a more significant role in the development of galaxies in the early universe than previously thought. Porphyrion existed in an era when the delicate filaments connecting galaxies, known as the cosmic web, were much closer together than they are now. This means that enormous jets like Porphyrion could reach a larger area of the cosmic web compared to jets in our nearby universe.
George Djorgovski, a co-author and a professor of astronomy and data science at Caltech, explains, “Astronomers think that galaxies and their central black holes evolve together, and a crucial factor in this process is that jets can distribute a large amount of energy, impacting the growth of the galaxies they inhabit as well as neighboring ones. This discovery reveals that their influence can extend much further than we previously believed.”
Discovering a Vast Population
The Porphyrion jet system is the largest identified so far in a sky survey that has uncovered an astonishing number of these faint megastructures: over 10,000. This extensive collection of gigantic jets was identified using Europe’s LOFAR (LOw Frequency ARray) radio telescope.
Prior to LOFAR’s observations, while several large jet systems were known, they were considered rare, and it was believed they were generally smaller than the thousands of systems now found by the radio telescope.
Martin Hardcastle, the second author of the study and a professor of astrophysics at the University of Hertfordshire in England, states, “We knew giant jets existed before our campaign began, but we were unaware there would be so many. Typically, with a new observational tool like LOFAR’s broad field view and exceptional sensitivity to extended structures, we discover something new; however, it was still quite thrilling to find so many of these objects.”
Back in 2018, Oei and his team started using LOFAR to investigate not black hole jets, but rather the cosmic web of filaments that weave through the space between galaxies. While examining the radio images for these faint filaments, they began noticing incredibly long jet systems.
Oei, who is also connected with Leiden Observatory in the Netherlands, reflects, “We were quite astonished when we first discovered the giant jets. We had no idea that there were this many.”
To systematically search for more undiscovered jets, the team carefully analyzed the radio images, employed machine-learning tools to detect hints of the lurking jets, and involved citizen scientists worldwide to help examine the images further. A paper detailing their recent findings of giant outflows, which include over 8,000 jet pairs, has been accepted for publication in the journal Astronomy & Astrophysics.
Hiding in the Past
The team used the Giant Metrewave Radio Telescope (GMRT) in India, along with additional data from a project called the Dark Energy Spectroscopic Instrument (DESI) from Kitt Peak National Observatory in Arizona, to identify the galaxy from which Porphyrion originated. Their observations identified the jets’ source as a massive galaxy approximately ten times the mass of our Milky Way.
They then utilized the W. M. Keck Observatory in Hawaii to confirm that Porphyrion is located 7.5 billion light-years away from Earth. Oei notes, “Until now, these giant jet systems seemed to be a phenomenon of the recent universe. If such distant jets can extend to the scale of the cosmic web, this indicates that black hole activity may have influenced every area of the universe at some point in cosmic history.”
Keck’s observations also indicated that Porphyrion originated from what is termed a radiative-mode active black hole, contrasting with a jet-mode black hole. When supermassive black holes activate—primarily when their intense gravitational forces pull and heat surrounding materials—they are believed to either emit energy as radiation or through jets. Radiative-mode black holes were more prevalent in the earlier stages of the universe, while jet-mode black holes are more common today.
The origin of Porphyrion from a radiative-mode black hole surprised astronomers since it was previously unknown that this mode could produce such immense and powerful jets. Furthermore, because Porphyrion exists in the distant universe, a place rich with radiative-mode black holes, this discovery suggests that many more gigantic jets may still be waiting to be uncovered.
“We might only be seeing the beginning of what’s out there,” Oei mentions. “Our LOFAR survey has only examined 15 percent of the sky, and since identifying these giant jets can be challenging, it’s likely that many more of these colossal structures exist.”
Lingering Questions
It remains unclear how these jets can extend so far beyond their host galaxies without becoming unstable. Hardcastle, an expert in the physics of black hole jets, remarks, “Martijn’s research indicates that there isn’t anything particularly unique about the environments surrounding these giant sources that causes them to achieve such significant sizes. I believe we need an exceptionally long-lasting and stable accretion event around the supermassive black hole to enable it to remain active for about a billion years and ensure that the jets maintain their direction throughout that period. Our findings about the large number of giants suggest that this must be a relatively common occurrence.”
Looking ahead, Oei wants to delve deeper into understanding how these gigantic structures affect their surroundings. The jets distribute cosmic rays, heat, heavy elements, and magnetic fields throughout the intergalactic space. Oei is particularly interested in how much these massive jets spread magnetism. “The magnetism present on our planet is essential for life, so understanding its origins is vital,” he states. “We know that magnetism permeates the cosmic web, then transitions into galaxies and stars, eventually reaching planets. The key question is: Where does it begin? Have these giant jets played a role in propagating magnetism throughout the cosmos?”
