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There are plenty of mind-bendingly large structures in our Universe.
But when it comes to galaxies, two discoveries over the past five years have given the word 'massive' a whole new meaning.
Case in point, one of the biggest structures of galactic origin detected by humans in radio waves: Porphyrion.
Named after the king of giants in Greek mythology, Porphyrion is 7.5 billion light-years away, so it's not exactly in our galactic neighborhood.
It was first detected in 2024, when researchers picked up radio signals from a pair of jets being blasted out by the black hole presumably at the galaxy's center.
Those jets spanned an incomprehensible 23 million light-years (or 7 megaparsecs) across, blasting out from the black hole at the center of the galaxy.
That's roughly the distance of 140 Milky Ways stacked end to end.
Let that sink in. 140 galaxies. End to end.
"Porphyrion shows that small things and large things in the Universe are intimately connected. We are seeing a single black hole that produces a structure of a scale similar to that of cosmic filaments and voids," astronomer Martijn Oei of Leiden University in the Netherlands and Caltech, lead author on the discovery paper published in Nature, told ScienceAlert back in 2024.
"If we shrink the jets to the size of the Earth and the black hole accordingly, the black hole would have the size of 0.2 millimeters: the size of an amoeba or a mite on your skin. So these giant jets are incredible: they are as if a single amoeba was able to generate a powerful fountain of energy the size of the entire Earth!" – Astronomer Martijn Oei
The jets are known as 'galactic lobes' – structures of high-energy plasma and gas that extend well beyond a galaxy's visible disc.
When it was discovered, Porphyrion became the Universe's biggest known structure of galactic origin.
(Since then, another radio galaxy – TXS 0033+252 – has been identified spanning 26 million light-years, effectively claiming the record.)
At the time, Porphyrion took the title from another structure discovered by the same team: Alcyoneus.
Lurking some 3 billion light-years away, Alcyoneus is a giant radio galaxy reaching 5 megaparsecs into space, or 16.3 million light-years long.
Both structures were picked up in data collected by the LOw Frequency ARray (LOFAR) in Europe, an interferometric network consisting of around 20,000 radio antennas, distributed throughout 52 locations across Europe.
Their discoveries are providing new insight into the evolution and growth of galaxies in the early Universe – and are also helping us understand the intergalactic medium that drifts in the voids of space.
That's because we still don't actually know very much about these giant radio galaxies.
"They consist of a host galaxy (that's the cluster of stars orbiting a galactic nucleus containing a supermassive black hole), as well as colossal jets and lobes that erupt forth from the galactic center," Michelle Starr wrote for ScienceAlert in 2022.
Those jets and lobes interact with the intergalactic medium and somehow act as a synchrotron to accelerate electrons that produce radio emission.
While the phenomenon is common – even the Milky Way has radio lobes – what we don't really have a good handle on is why, in some galaxies, they grow to absolutely gargantuan sizes, on megaparsec scales.
The jets of Alcyoneus and Porphyrion in particular present several challenges.
The first is that, in order to generate such huge jets, the black hole needs to be feeding continuously – which implies a huge reservoir of material – for a period of about a billion years.
That's not typical, although obviously it's not impossible, because we're looking at the results.
The other is the length of the jets. The team looked back 7.5 billion years into the past to see Porphyrion, a time when intergalactic space was more dense than it is today. The longer a jet grows, the more unstable it becomes. Once an instability is introduced, the jet should fall apart.
How the jets of Alcyoneus and Porphyrion remained stable over such vast distances of space-time is an ongoing mystery.
But they could be giving us some clues about how the Universe is structured.
Spanning across every epoch of space-time is a structure called the cosmic web – a huge system of filaments, consisting of dark matter that gravitationally corrals galaxies; the hubs of galaxy clusters where these filaments meet; and the huge voids between the filaments.
The length of Porphyrion's jets, according to the team's calculations, is about 66 percent of the radius of the void within which Porphyrion sat at that point in time.
As Starr wrote in 2024 for ScienceAlert, this suggests that these mega-jets could be playing a role in shaping the cosmic web.
They could, the researchers say, be responsible for the strangely high temperatures detected in voids, and the magnetic field structures found therein. These features could have been placed there by the jets.
This is especially interesting, because the black hole is of a kind seen quite often in the early Universe. One idea is that there were a lot more huge jets in the early Universe than we realize, playing a significant role in sculpting the cosmic web.
Even later, the research team believes, the cosmos could be teeming with such galaxies.
"I think galaxies with giant jets are more common than we realize," Oei told ScienceAlert.
"That is because giant jet systems become harder to observe when they grow larger. They're also harder to observe in the more distant Universe.
"So the number of giant jets detected at the moment is limited by our instrumental capabilities. Once the instruments improve in the coming few years, I expect that many more galaxies with giant jets will be found."
Indeed, ongoing surveys of the sky in radio waves suggest even larger structures may already be lurking in the data.
So the record for the biggest structure of galactic origin may not stand for long.
The research has been published in Astronomy & Astrophysics, and Nature.
This article was fact-checked by Michael Irving and edited by Peter Dockrill. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
