Longest jets in the universe discovered – giant particle streams as long as 140 Milky Way galaxies in a row

Observations show that supermassive black hole jets can extend far beyond their host galaxies. “If these active jets persist for a long time, they could become the largest galactic-generated structures in the Universe,” explains Martijn Ooi of Leiden University and colleagues. In 2022, they discovered the longest radio jet to date. Named after the mythical giant Alkyonius, the structure is about 16 million light-years long.

The largest of all aircraft giants

But now there is a new record holder. Ooi and his team tracked it down when they systematically searched for large radio jets using the LOFAR radio telescope network. “When we started this, we had no idea we would find this many,” says co-author Martin Hardcastle of the University of Hertfordshire. To their surprise, the astronomers identified more than 11,000 of these massive streams of particles, in a section covering just 15 percent of the entire sky.

The team discovered the largest radio jet about 7.5 billion light-years away: two narrow tails glowing in radio light, emanating from a large galaxy about ten times the mass of the Milky Way. Closer measurements showed that the radio jets were about 23 million light-years long. Named after another Greek giant, these Porphyry jets are even longer than the previous record holder, Alcyoneus.

As long as the 140 Milky Way galaxies are connected to each other

“The length of this jet pair is equivalent to 140 Milky Way diameters,” explains Ooi. “Our galaxy would be just a tiny speck in these two giant explosions.” Thus, the Porphyry jets exceed the previously assumed upper limit for such outflows from black holes, the astronomers explain: Because only shorter radio jets were previously known, lengths of more than 16 million light-years were assumed to be nonexistent. But Porphyry now teaches otherwise.

And that's not all: to be able to reach so far into space and remain stable, these dual particle streams must be fueled by enormous energy. “The energy of the porphyry jets is about 8 × 10⁵⁵ “The joules are being transferred to the intergalactic medium, which is similar to the energy released during the collision of galaxy clusters,” he said. “This makes this outflow one of the most energetic events after the Big Bang in the entire cosmic web.”

The constitutive factor of the cosmic web?

The discovery of these standard jets is also relevant for another reason: “Until now, such giant jet systems seemed to be a phenomenon of the universe today,” explains Oye. But the porphyry jets were already present when the universe was less than half its current age. Back then, the universe was much younger than it is today, and its major structures, including the filaments of the cosmic web, were much closer together.

A giant outflow like Porphyry would have been long enough at that time to reach from one filament of the cosmic web to another. Such outflows could cross voids, which are huge, thin regions of the universe. However, this means: “If such outflows reach the size of the cosmic web, it is possible that every place in the universe would be affected by this black hole activity at some point,” explains Oye.

The discovery of porphyry underscores the importance of active black holes and their outflows in the evolution of large-scale cosmic structures as a whole, according to astronomers.

Jet builder in unexpected situation

Also interesting: The porphyry jets come from a supermassive black hole, which, according to current theory, should not produce any jets, especially those that far away. Because astronomers usually distinguish between two states of active black holes. In the so-called radiatively efficient (RE) mode, active galactic nuclei (AGNs) release their energy mainly in the form of radiation. This mode was particularly common in the early Universe in galaxies with intense star formation. It was also previously considered unsuitable for large jets.

In contrast, black holes are in radiatively inefficient (RI) mode. They are the originators of almost all known giant radio jets, including Alkyonius. But as Ooi and his team discovered, the porphyry jets do not come from a black hole in RI mode, but rather originate in a galaxy nucleus in the radiation-dominated RE mode. “Porphyry suggests that nuclei of regenerating galaxies can generate such outflows at least as efficiently as RI-AGNS in the local Universe,” the astronomers say.