The largest galaxy ever seen, on radio: Alcyoneus

Scientists have named the largest galaxy to date, detected by radio astronomy, Alcyoneus. In Greek mythology, Alcyoneus was a giant, an offspring of Gaia, and immortal in his native land. Image via Leiden University.

Astronomers discover the largest galaxy yet, on radio

Astronomers said last week (February 16, 2022) that they had found the largest galaxy ever seen, not in visible light, but via radio astronomy. This galaxy spans 16 million light-years. Some 100 Milky Way galaxies placed end to end would roughly equal the length of this newly discovered galaxy. Scientists have named the new galaxy Alcyoneus, for the giant from Greek myth. They said it’s so large it could provide information about the Cosmic Web – the observed network of filaments and clusters, made up of galaxies, and the great voids between them – that astronomers observe all around us in the modern universe.

Alcyoneus lies relatively close to galaxies, only 3 billion light-years away. It covers an area of ​​our sky the size of the full moon.

Martijn Oei from Leiden University led the research team. Their job was to reprocess existing images from the Low Frequency Array (LOFAR) telescope to bring the giant out of obscurity. You can read a pre-print of their article on arXiv dated February 11, 2022. The article is accepted for publication in the journal Astronomy and astrophysics.

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The jets of Alcyoneus

Alcyoneus derives its enormous size from two enormous jets emanating from the galaxy’s central supermassive black hole. The statement from the astronomers read:

A supermassive black hole lurks at the center of many galaxies. [It] slows the birth of new stars and therefore strongly influences the life cycle of the galaxy as a whole. Sometimes this leads to tumultuous scenes: the black hole can create two jet streams that catapult the building material of baby stars out of the galaxy at almost the speed of light [186,000 miles per second, or 300,000 km/sec]. In this violent process, stardust heats up so much that it dissolves in plasma and glows in radio light.

These astronomers said it was the first time anyone had seen a single galaxy with such a large structure (the two jets).

Green circle of grass with moats and black squares scattered inside.
The LOFAR (Low Frequency Array) telescope is located in a swampy Dutch “radio dark” nature reserve, “where your smartphone deliberately loses the signal”. Astronomers used this telescope to find the 16 million light-year-long radio galaxy now known as Alcyoneus. Image via LOFAR.

Probe the cosmic web

Scientists think plasma plumes or jets from Alcyoneus could help provide information about what’s called the Cosmic Web. They said in their statement:

The Cosmic Web is another name for the contemporary adult universe, which resembles a network of threads and knots that astronomers call filaments and clusters, respectively. Filament and cluster galaxies are clearly visible themselves, but detecting the medium between the galaxies [voids in the Cosmic Web] only succeeded in clusters, with a few exceptions. Could Alcyoneus change that?

They went on to say:

Because Alcyoneus, like the Milky Way, inhabits a filament, its plumes feel a headwind as they move through the medium. This subtly changes the direction and shape of the plumes: they perform a slow dance with an invisible partner. For many years scientists have proposed that the shapes and pressures in the plumes of radio galaxies might be related to the properties of filaments, but never before have they found an example where this connection is as plausible as with Alcyoneus.

In other words, the plumes of Alcyoneus are so large and rarefied that the surrounding environment can mold them relatively easily.

Streamers and bundles of light green threads on a dark blue background.
Simulation of the universe and the filaments, clusters and voids that make up the first cosmic web in the distant universe. The image shows light emitted by hydrogen atoms in the cosmic web in a region about 15 million light-years across. In addition to the very weak emission of intergalactic gas, several point sources are visible: these are galaxies in the process of forming their first stars. Image via Jeremy Blaizot / The SPHINX Project.

Conclusion: Astronomers have discovered the largest radio galaxy to date. It would take 100 Milky Way galaxies placed end to end to equal Alcyoneus and his jets. The jets are so tenuous that astronomers can see subtle changes in them as they move through the intergalactic medium. Astronomers hope this subtle movement will help them understand the voids in the cosmic web.

Source: The discovery of a galaxy radio of at least 5 Mpc

Via Leiden University

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