Porphyrion: the largest black hole jets, 140 Milky Ways long

The largest black-hole jets ever seen, a pair nicknamed Porphyrion, span about 23 million light-years (7 megaparsecs) tip to tip, the length of roughly 140 Milky Ways laid end to end. A team led by Caltech’s Martijn Oei reported them in Nature in September 2024.

How big are the biggest black-hole jets?

Porphyrion is now the record at about 140 Milky Ways, ahead of Alcyoneus, found by the same team in 2022, at about 100, and far beyond Centaurus A, the nearest giant jet system to Earth, at about 10. For some 50 years, the known size range of jet pairs topped out at 4.6–5.0 megaparsecs; Porphyrion pushes it to about 7.

“The Milky Way would be a little dot in these two giant eruptions.” — Martijn Oei, Caltech

It is not a lone freak either, just the biggest of more than 10,000 giant jet pairs turned up by Europe’s LOFAR radio telescope, objects once thought rare. Co-author Martin Hardcastle suggests reaching such sizes takes an unusually stable feeding episode lasting about a billion years, with the jets holding their aim throughout.

Why did Porphyrion surprise astronomers?

First, because of its age. The jets pour from a supermassive black hole in a galaxy about 10 times as massive as ours, 7.5 billion light-years away, so we see it as it was when the universe was 6.3 billion years old, less than half its present age. Giant jets had seemed a feature of the recent cosmos.

The engine was a surprise too. Keck Observatory spectra showed Porphyrion’s black hole is a radiative-mode type, one astronomers did not know could launch outflows this powerful. Because such black holes were common in the young universe, many more colossal jets may be waiting to be found.