Alcyoneus is a massive radio galaxy lurking some 3 billion light-years from us, reaching a distance of 5 megaparsecs into space. This is 16.3 million light-years long and represents the largest known structure of galactic origin.

The finding highlights our lack of understanding of these behemoths and what’s driving their phenomenal growth. But it could provide a route to a better understanding, not just for massive radio galaxies, but also for the intergalactic medium drifting through the open spaces.

Giant radio galaxies are another mystery in a universe full of enigmas. They consist of a host galaxy (a cluster of stars orbiting the core of a galaxy containing a supermassive black hole) and huge jets and lobes ejected from the galaxy’s center.

These jets and lobes interact with the interstellar medium, acting as synchrotrons to accelerate electrons that produce radio emissions.

We’re pretty sure we know what’s causing the jet: an active supermassive black hole at the center of our galaxy. We call a black hole “active” when it devours (or “accretions”) matter from the huge surrounding disk of matter.

Not all the matter in the accretion disk swirling into the active black hole will necessarily go beyond the event horizon. A small portion of it is somehow directed from inside the accretion disk to the poles, where it is ejected into space at a large fraction of the speed of light in the form of jets of ionized plasma.

These jets can travel great distances and then spread out into huge radio lobes.

This process is normal. Even the Milky Way has radio waves. What we really don’t understand is why they grow to absolutely massive megaparsec sizes in some galaxies. Known as giant radio galaxies, the most extreme examples may hold the key to understanding what drives their growth.

“If certain features of host galaxies are important for the growth of giant radio galaxies, then the hosts of the largest giant radio galaxies may also have them,” said the researchers, led by astronomer Martijn Oei of the Leiden Observatory in the Netherlands, in their prediction. Explanation in print – Papers accepted for publication in Astronomy and Astrophysics.

“If there are certain large-scale environments that are very favorable for the growth of giant radio galaxies, then the largest giant radio galaxies are likely to exist there.”

The team looked for these outliers in data collected by the European Low Frequency Array (LOFAR), an interferometric network of around 20,000 radio antennas spread across 52 sites in Europe.

They reprocessed the data through a new pipeline, removing compact radio power sources that could interfere with the detection of diffuse radio lobes and correcting optical distortions.

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