Largest galaxy yet seen, in radio: Alcyoneus

Largest galaxy yet: Small oblong light at center with two larger orange-yellow lobes, one on each side.
Scientists have named the largest galaxy yet, as detected via radio astronomy, Alcyoneus. In Greek mythology, Alcyoneus was a giant, an offspring of Gaia, and immortal while on his native land. This image shows 2 lobes of radio emission in orange, and the jets that caused them, shooting out from the center of the galaxy as a diagonal line. Image via the University of Leiden.

Astronomers find largest galaxy yet, in radio

Astronomers said last week (February 16, 2022) they’ve found the largest galaxy yet seen, not in visible light, but via radio astronomy. This galaxy spans a mind-boggling 16 million light-years. Some 100 Milky Way galaxies laid end to end would about equal the length of this newly discovered galaxy. Scientists have named the new galaxy Alcyoneus, for the giant of Greek myth. They said it’s so large that it might provide insights about the the cosmic web – the observed network of filaments and clusters, made of galaxies, plus the great voids between them – that astronomers observe all around us in the modern universe.

Alcyoneus lies relatively nearby as galaxies go, only 3 billion light-years away. It spans an area in our sky the size of the full moon.

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

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Alcyoneus’ jets

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

A supermassive black hole lurks in the center of many galaxies. [It] slows down 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 for 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, the stardust heats up so much that it dissolves into plasma and glows in radio light.

These astronomers said this is the first time anyone has seen a single galaxy with a structure (the two jets) this large.

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

Probing the cosmic web

Scientists think the plasma plumes or jets from Alcyoneus might help provide insights about what’s called the cosmic web. They said in their statement:

The cosmic web is another name for the contemporary, grown-up universe, that looks like a network of threads and nodes that astronomers call filaments and clusters, respectively. The galaxies in filaments and clusters are clearly visible themselves, but detecting the medium between galaxies has only been successful in clusters, barring a handful of exceptions. Could Alcyoneus change this?

They went on to say:

Because Alcyoneus, just like the Milky Way, inhabits a filament, its plumes feel a headwind while moving 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 of and pressures in the plumes of radio galaxies could relate to filament properties, but never before did they find an example where that connection is as plausible as with Alcyoneus.

In other words, Alcyoneus’ plumes are so big and rarefied that the surrounding medium can relatively easily mold them. Can astronomers detect changes? And if so, will those detections buoy our understanding of what lies between the galaxies?

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

Bottom line: Astronomers have discovered the largest radio galaxy yet. It would take 100 Milky Way galaxies laid end to end to equal Alcyoneus and its 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 radio galaxy of at least 5 Mpc

Via University of Leiden

February 20, 2022

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