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Researchers Discover the Largest Radio Galaxy Ever — At Least 16 Million Light-years Long

A research team headed by Martijn Oei, a Dutch Ph.D. student, has identified a radio galaxy — of at least 16 million light-years long — by a stroke of luck. The plasma plumes pair is the hugest structure made by a galaxy known so far. The result challenges some long-believed hypotheses about the evolution of radio galaxies.

Researchers Discover the Largest Radio Galaxy Ever—At Least 16 Million Light-years Long.
Part of LOFAR in Exloo, Drenthe. Image Credit: LOFAR.

A supermassive black hole lurking in the center of many galaxies slows down the new stars’ birth and, in turn, strongly impacts the galaxy’s whole lifecycle. At times, it can lead to unrestrained scenarios, where the black hole makes two jet streams, catapulting the baby stars’ building materials out of the galaxy at a speed close to that of light.

In this aggressive process, the stardust heats up so much that it dissolves into plasma and glows in the presence of radio light.

The global research team comprising scientists from Leiden (The Netherlands), Hertfordshire, Oxford (both UK) and Paris (France) has now gathered that light with the pan-European LOFAR telescope, for which the epicenter falls in a marshy Dutch “radio dark” nature reserve — where a smartphone drops signal deliberately.

Record Length

The image of the two plasma plumes is exceptional because scientists have never seen such a huge structure made by a single galaxy. The finding reveals that the sphere of influence of certain galaxies stretches far from their direct environment. How far, exactly, is hard to identify.

As astronomical images are captured from a single viewpoint (Earth), they do not have depth. As a consequence, researchers can only quantify a section of the radio galaxy length, which is a low evaluation of the complete length. However, even that lower bound, which is of more than 16 million light-years, is enormous and equated to a hundred Milky Ways in a row.

Visible with the Naked Radio Eye

As Earth does not hold a distinct space in the Universe, it was never possible that such a huge galactic structure would be found in everyone’s backyard. And, in fact, the radio giant is three billion light-years away from the Earth. In spite of that astounding distance, the giant looks as huge as the Moon in the sky - an indication that the structure had to have a record length.

The radio eyes of the LOFAR telescope were able to see the giant because of the fact the plumes were relatively dim. The researchers were suddenly able to observe the giant, by reprocessing a set of already present images in a way that subtle patterns stand out.

The Giant Alcyoneus

The scientists named the giant structure Alcyoneus, after the son of Ouranos — the Greek sky God. For authority over the cosmos, this giant mythological Alcyoneus fought with Heracles and other Olympians. A statue of Alcyoneus is sculpted in the world-popular Pergamon Altar, located in Berlin.

Ghostly Dance

The plumes of Alcyoneus likely show information about the usually elusive Cosmic Web filaments. The Cosmic Web is an alternate name for the current, evolved Universe, which resembles a network of nodes and threads that astrophysicists address as clusters and filaments, respectively.

The galaxies are clearly visible in filaments and clusters, but spotting the medium between galaxies has been successful only in clusters, excluding some exceptions. Can Alcyoneus modify this?

As Alcyoneus accommodates a filament, as the Milky Way does, its plumes sense a headwind while passing via the medium. This changes the shape and direction of the plumes subtly; they do a slow dance accompanying an invisible partner.

Scientists, for many years, have been proposing that the pressures in and shapes of the plumes of radio galaxies can relate to properties of filaments, but they have never before found an example where that association is as plausible as with Alcyoneus. Alcyoneus’ plumes are so huge and exclusive that they can be easily molded by the surrounding medium.

Black holes are Cosmic Mainstays

The Cosmic Web stays in its form because the heat pressure of the medium in filaments and clusters compensates for the attractive force of gravity. It has become evident in the past two decennia that the glowing stardust, which jet streams expel from galaxies, retains the Web warm. In such a way, the large-scale structure of our Universe is sustained by the central black holes in galaxies’ contribution.

As black holes are very tiny in comparison to filaments and clusters, the contribution is extra striking. It is as if Earth’s temperature is regulated by something as small as the size of a marble.

Mysterious Origin

What remains a mystery, for now, is the answer to “what has given Alcyoneus its record length?” At first, the researchers thought of an incredibly huge black hole to have a wide stellar population (with a lot of stardust) or extremely potential jet streams.

To their surprise, Alcyoneus seems to be below average on all these aspects in comparison to its siblings. The scientists intend to examine whether the radio galaxies' environments could explain the evolution of giants.


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