Astronomers discover largest black hole jets ever observed, shooting hot plasma far beyond their own host galaxy

Astronomers have spotted the largest pair of black hole jets ever observed, spanning a total of 23 million light-years. That’s equivalent to lining up 140 Milky Way galaxies back to back.

“This pair is not only the size of a solar system or a Milky Way; we’re talking about 140 Milky Way diameters in total,” says Martijn Oei, a postdoctoral researcher at Caltech and lead author of a paper. Nature An article reporting these results: “The Milky Way would be only a small point in these two giant eruptions.”

This jet-filled megastructure, nicknamed Porphyrion after a giant in Greek mythology, dates from a time when our universe was 6.3 billion years old, less than half its current age of 13.8 billion years. These powerful jets, with a total force equivalent to that of trillions of suns, shoot out from above and below a supermassive black hole at the heart of a distant galaxy.

Before Porphyrion was discovered, the largest confirmed jet system was Alcyonea, also named after a giant in Greek mythology. Alcyonea, which was discovered in 2022 by the same team that discovered Porphyrion, spans the equivalent of about 100 Milky Ways. For comparison, the famous Centaurus A jets, the closest major jet system to Earth, span 10 Milky Ways.

This latest discovery suggests that these giant jet systems may have had a greater influence on galaxy formation in the early universe than previously thought.

Porphyrion existed in a distant time when the thin filaments that connect and power galaxies, known as the cosmic web, were closer together than they are today. That means huge jets like Porphyrion passed through more of the cosmic web than jets in the local universe.

“Astronomers think that galaxies and their central black holes co-evolve, and a key aspect of this theory is that jets can radiate enormous amounts of energy that affect the growth of their host galaxies and other galaxies near them,” said co-author George Djorgovski, a professor of astronomy and data science at Caltech. “This discovery shows that their effects can extend much further than we thought.”

Revealing a vast population

The Porphyrion jet system is the largest ever discovered in a sky survey that revealed a shocking number of faint megastructures: more than 10,000. This massive population of gargantuan jets was discovered using Europe’s LOFAR (LOw Frequency ARray) radio telescope.

Although hundreds of large jet systems were known before LOFAR observations, they were thought to be rare and on average smaller than the thousands of systems discovered by the radio telescope.

“Giant jets were known before we started the campaign, but we had no idea there would be so many of them,” says Martin Hardcastle, second author of the study and professor of astrophysics at the University of Hertfordshire in England.

“Usually when we gain a new observational capability, like the combination of a wide field of view and very high sensitivity to the extended structures of LOFAR, we discover something new, but it was still very exciting to see so many of these objects emerge.”

In 2018, Oei and his colleagues began using LOFAR to study not black hole jets, but the cosmic network of thin filaments that crisscross the space between galaxies. As they scanned the radio images for faint filaments, the team began to notice several surprisingly long jet systems.

“When we discovered these giant jets, we were very surprised,” says Oei, who also works at the Leiden Observatory in the Netherlands. “We had no idea there were so many of them.”

To systematically search for other hidden jets, the team inspected radio images with the naked eye, used machine learning tools to scan the images for signs of impending jets, and enlisted the help of citizen scientists around the world to examine the images in more detail.

A paper describing their most recent batch of giant jets, containing more than 8,000 jet pairs, has been accepted for publication in the journal Astronomy and astrophysics.

Hiding in the past

To find the galaxy where Porphyrion originated, the team used the Giant Meteorological Radio Telescope (GMRT) in India and auxiliary data from a project called the Dark Energy Spectroscopic Instrument (DESI), which operates from the Kitt Peak National Observatory in Arizona. The observations helped pinpoint the jets’ focus, a massive galaxy about 10 times more massive than our Milky Way.

The team then used the W. M. Keck Observatory in Hawaii to show that Porphyrion is 7.5 billion light-years from Earth. “Until now, these giant jet systems seemed to be a phenomenon of the recent universe,” Oei says. “If distant jets like these can reach the scale of the cosmic web, then every place in the universe could have been affected by black hole activity at some point in cosmic time,” Oei says.

Keck’s observations also revealed that Porphyrion emerged from what is called a radiatively active black hole, as opposed to a jet-mode black hole.

When supermassive black holes become active—that is, when their immense gravitational forces attract and heat surrounding matter—they are thought to emit energy in the form of radiation or jets. Radiative black holes were more common in the early or distant universe, while jet black holes are more common in the present-day universe.

The fact that Porphyrion originated from a black hole in radiative mode came as a surprise, as astronomers had not known that this mode could produce such huge and powerful jets. Furthermore, since Porphyrion is in a distant universe where black holes in radiative mode are abundant, this discovery implies that there may be many more colossal jets yet to be discovered.

“We may be just seeing the tip of the iceberg,” Oei says. “Our LOFAR survey only covered 15 percent of the sky. And most of these giant jets are probably hard to spot, so we think there are a lot more of these behemoths in the sky.”

Mysteries in progress

It is still unclear how these jets can extend so far beyond their host galaxies without destabilizing. “Martijn’s work has shown us that there is nothing particularly special about the environment of these giant sources that causes them to reach such sizes,” says Hardcastle, an expert in black hole jet physics.

“My interpretation is that we need an exceptionally long and stable accretion event around the central supermassive black hole to allow it to be active for so long (about a billion years) and to ensure that the jets continue to point in the same direction for that long. What we learn from the large number of giants is that this must be a relatively common phenomenon.”

As a next step, Oei wants to better understand how these megastructures influence their environment. Jets propagate cosmic rays, heat, heavy atoms and magnetic fields into the space between galaxies. In particular, Oei wants to find out to what extent giant jets propagate magnetism.

“The magnetism of our planet allows life to develop, so we want to understand how it came about,” he explains. “We know that magnetism permeates the cosmic web, then spreads out into galaxies and stars, and eventually to planets, but the question is: where does it start? Did these giant jets spread magnetism across the cosmos?”