Caltech-led astronomers using W.M. Keck Observatory report spotting biggest pair of black hole jets ever seen

Black hole jets spanning 23 million light-years total length in outer space have been discovered by a Caltech-led team of astronomers using the W. M. Keck Observatory atop Maunakea on Hawaiʻi Island. The jets of hot plasma are the largest ever observed, according to the scientists.

“Nicknamed Porphyrion after the mythological Greek giant, this powerful, colossal jet system spans 23 million light-years, a distance equivalent to lining up 140 Milky Way galaxies back-to-back,” according to an announcement of the discovery. Porphyrion was located 7.5 billion light-years from Earth.

The study was published online Thursday in the journal Nature.

“This pair is not just the size of a solar system, or a Milky Way; we are talking about 140 Milky Way diameters in total,” said Martijn Oei, a Caltech postdoctoral scholar and lead author of the new study. “The Milky Way would be a little dot in these two giant eruptions.”

The jet megastructure dates to a time when the universe was 6.3 billion years old, or less than half its present age of 13.8 billion years. These fierce outflows — with a total power output equivalent to trillions of suns — shoot out from above and below a supermassive black hole at the heart of a remote galaxy.

Prior to Porphyrion’s discovery, the largest confirmed jet system was Alcyoneus, also named after a giant in Greek mythology. Alcyoneus, which was discovered in 2022 by the same team that found Porphyrion, spans the equivalent of around 100 Milky Ways. For comparison, the well-known Centaurus A jets, the closest major jet system to Earth, spans 10 Milky Ways.

The latest finding suggests that these giant jet systems may have had a larger influence on the formation of galaxies in the young universe than previously believed. Porphyrion existed during an early epoch when the wispy filaments that connect and feed galaxies, known as the cosmic web, were closer together than they are now. That means enormous jets like Porphyrion reached across a greater portion of the cosmic web compared to jets in the local universe.

“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, professor of astronomy and data science at Caltech. “This discovery shows that their effects can extend much farther out than we thought.”

To find the galaxy from which Porphyrion originated, the team used the Giant Metrewave Radio Telescope in India along with ancillary data from a project called Dark Energy Spectroscopic Instrument, which operates from Kitt Peak National Observatory in Arizona. The observations pinpointed the home of the jets to a hefty galaxy about 10 times more massive than our Milky Way. 

The team then used the Keck Observatory to show that Porphyrion is 7.5 billion light-years from Earth.

“Up until now, these giant jet systems appeared to be a phenomenon of the recent universe,” Oei said. “If distant jets like these can reach the scale of the cosmic web, then every place in the universe may have been affected by black hole activity at some point in cosmic time.”

Keck Observatory’s Low Resolution Imaging Spectrometer also revealed that Porphyrion emerged from a radiative-mode active black hole, as opposed to one that is in a jet-mode state. When supermassive black holes become active — in other words, when their immense forces of gravity tug on and heat up surrounding material — they are thought to either emit energy in the form of radiation or jets. Radiative-mode black holes were more common in the young, or distant, universe, while jet-mode ones are more common in the present-day universe. 

The fact that Porphyrion came from a radiative-mode black hole came as a surprise because astronomers did not know this mode could produce such huge and powerful jets. What is more, because Porphyrion lies in the distant universe where radiative-mode black holes abound, the finding implies there may be a lot more colossal jets left to be found.

How the jets can extend so far beyond their host galaxies without destabilizing is still unclear.

“Martijn’s work has shown us that there isn’t anything particularly special about the environments of these giant sources that causes them to reach those large sizes,” said Martin Hardcastle, an expert in the physics of black hole jets. “My interpretation is that we need an unusually long-lived 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 keep pointing in the same direction over all of that time. What we’re learning from the large number of giants is that this must be a relatively common occurrence.”

As a next step, Oei wants to better understand how these megastructures influence their surroundings. The jets spread cosmic rays, heat, heavy atoms, and magnetic fields throughout the space between galaxies.

Oei is specifically interested in finding out the extent to which giant jets spread magnetism.

“The magnetism on our planet allows life to thrive, so we want to understand how it came to be,” he said. “We know magnetism pervades the cosmic web, then makes its way into galaxies and stars, and eventually to planets, but the question is: Where does it start? Have these giant jets spread magnetism through the cosmos?”