Astronomers have discovered the biggest radio galaxy yet discovered.

By chance, a team lead by Dutch Ph.D. student Martijn Oei discovered a radio galaxy at least 16 million light-years long. The pair of plasma plumes is the galaxy’s greatest structure to date. The discovery refutes several long-held assumptions concerning the evolution of radio galaxies.

A supermassive black hole lurks in the heart of many galaxies, slowing the production of new stars and so having a large impact on the galaxy’s overall lifetime. This can result in chaotic scenes: the black hole can generate two jet streams, which hurl the building material for young stars out of the galaxy at almost the speed of light. The stardust warms up so rapidly during this violent process that it melts into plasma and glows in radio light. The light was captured by an international team of researchers from Leiden, Hertfordshire, Oxford, and Paris using the pan-European LOFAR telescope, whose epicenter is in a marshy Dutch radio dark’ nature reserve where your smartphone intentionally loses service.

Record length

The image of the two plasma plumes is noteworthy because scientists have never seen a structure this large created by a single galaxy before. The research demonstrates that the sphere of the effect of some galaxies extends far beyond their immediate surroundings. How far are we talking? That is difficult to ascertain. Astronomical images are acquired from a single point of view (Earth) and hence lack depth. As a result, scientists can only measure a portion of the radio galaxy’s length, providing a conservative estimate of the whole length. Even still, the lowest bound of more than 16 million light-years is enormous, similar to a hundred Milky Ways in a row.

The naked radio eye can see it

Because Earth does not have a unique position in the cosmos, it was never anticipated that such a massive cosmic structure would be found in our own backyard. And it’s true: the radio behemoth is three billion light-years away. Despite the incredible distance, the mammoth loom was as huge as the moon in the sky, indicating that the building had to be a record length. Because the plumes are quite weak, the radio eyes of the LOFAR observatory only just detected the monster. The scientists were able to identify the mammoth by reprocessing a batch of old photographs in such a manner that minor patterns shone out.

The gigantic edifice was dubbed Alcyoneus by the researchers after Ouranos, the Greek primal deity of the sky. This legendary Alcyoneus was a giant who battled Heracles and other Olympians for control of the cosmos. A sculpture of this Alcyoneus may be seen at Berlin’s world-famous Pergamon Altar.

Ghostly dance

The plumes of Alcyoneus may give information on the usually mysterious strands of the Cosmic Web. The Cosmic Web is another name for the modern, grown-up cosmos, which resembles a network of threads and nodes known as filaments and clusters, respectively. The galaxies in filaments and clusters are readily visible, however finding the medium between galaxies has only been achieved in clusters (with a few outliers). Could Alcyoneus make a difference?

Because Alcyoneus, like the Milky Way, lives in a filament, its plumes experience a headwind as they move across the medium. This gently alters the plume’s path and shape: they do a leisurely dance with an invisible companion. Scientists have argued for many years that the shapes and pressures of radio galaxies’ plumes may be related to filament features, but never before have they seen a case where that relationship is as feasible as with Alcyoneus. Alcyoneus’ plumes are so large and rarefied that the surrounding medium may readily mold them.

Black holes are cosmic stalwarts

Because the thermal pressure of the medium in filaments and clusters compensates for the attraction pull of gravity, the Cosmic Web maintains its shape. The shimmering stardust that jet streams expel from galaxies has been proven to keep the Web toasty for the last two millennia. In this approach, galaxies’ center black holes help to maintain the universe’s large-scale structure. This is especially significant given how tiny black holes are in comparison to filaments and clusters. It’s as if a marble-sized object controls the Earth’s temperature.

Mysterious origin

For the time being, it is unknown what gave Alcyoneus his record length. The researchers originally considered a giant black hole, a large stellar population (and hence a lot of stardust), or extremely intense jet streams. Surprisingly, when compared to its smaller sisters and brothers, Alcyoneus appears to be below average in all of these areas. The researchers will now study if the surroundings of radio galaxies may instead explain the rise of giants in the future.

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