Astronomers Describe Their Discovery of “Alcyoneus” -The Largest Galaxy in the Universe


Galaxy Alcyoneus


One of the main questions the discovery of a giant radio galaxy (GRG) –active galactic nuclei that are very luminous at radio wavelengths– named Alcyoneus, after the son of the primordial Greek god of the sky, Ouranos, directly triggered is: “Why has Alcyoneus grown to such a large size,” asked astronomer Reinout van Weeren at the Leiden Observatory in an email to The Daily Galaxy. “The host galaxy of Alcyoneus and its central massive black hole have properties that are rather typical for radio galaxies,” van Weeren explained. “So Alcyoneus is not really special in this way. It is likely that Alcyoneus’ cosmic environment has played a role in allowing it to grow to its enormous size.

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“The region where Alcyoneus is located,” van Weeren continued, “must have a density that is low enough so that its giant lobes keep expanding. Therefore a next step in the research is to obtain better measurements of Alcyoneus’ environment. In addition, it is possible that there are even larger cousins of Alcyoneus lurking in our radio surveys. We are currently sifting through the survey data to find these.”

Researchers at the Leiden Observatory in the Netherlands made the discovery by analyzing the second data release (DR2) as part of the LOFAR Two-meter Sky Survey (LoTSS). The GRG extends beyond its host elliptical galaxy J081421.68+522410, which has a stellar mass of approximately 240 billion solar masses and a supermassive black hole (SMBH) mass of about 400 million solar masses. It is roughly 3 billion light years away from Earth and almost 240 billion times the size of our sun.

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GRGs–Rare Objects

GRG, rare objects grown in low-density environments, are radio galaxies with an overall projected linear length exceeding at least 2.3 million light years. To date, about a thousand GRGs have been detected, out of which only ten exceed 10 million light years in size, the largest being J1420-0545—with a projected proper length of approximately 16 million light years. GRGs and also the rest of the megaparsec-scale radio galaxies are assumed to be the largest single-galaxy–induced phenomena in the universe.

160 Times the Size of the Milky Way 

The discovery spans across 16.3 million light year making it four times the size of IC 1101and 160 times the size of the Milky Way galaxy, which makes it the largest galaxy ever discovered. The previous title was held by galaxy IC 1101, which spanned across 3.9 million light-years.

The discovery was possible due to a supermassive blackhole that the galaxy fosters at its center, which sucks an enormous amount of matter and then spits out gigantic jets of plasma, close to the speed of light. These plumes of plasmas–by far the largest ever discovered.– subside after traveling millions of light years before converting into plumes of radio waves. The astronomers conjectured that being surrounded by a low-density environment resulted in the GRG’s plasma jets to expand unprecedentedly.

Martijn Oei, the lead astronomer of the project wrote in their preprint paper that if the characteristics of the existing host galaxy are an important cause for giant radio galaxy growth, then the hosts of the largest giant radio galaxies are likely to possess them.

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The Last Word –“We may have a new record here!”

“When I saw Alcyoneus’ plumes in our images,” Martijn Oei wrote in an email to The Daily Galaxy, “I did not immediately realize this was a special discovery. My knowledge of radio galaxies wasn’t great at that point, and I had only just learnt how long the largest recorded example was. I did a preliminary back-of-the-envelope calculation and out came: 5 megaparsecs. At that point I got really excited and restlessly checked the steps again to see whether what I had done was correct… Amazingly the result stood up! Then I realized I might really be up to something. I emailed a radio galaxy expert in the UK, Martin Hardcastle, who wrote back:'”So I’d say this is pretty secure. Congratulations, we may have a new record here.’ I couldn’t believe it!!

“The most direct question that the discovery triggered,” Oei wrote in his email to The Daily Galaxy is: how did this radio galaxy grow to such a colossal scale? (The lobes are megaparsecs away from each other, which means they are far enough apart to feel substantial Hubble expansion!) To uncover why some radio galaxies grow to the megaparsec-scale is a key question in the research of radio galaxies, and the key question in the research of giant radio galaxies. We noted that a very extreme example, such as Alcyoneus, could lay bare the main drivers of radio galaxy growth: to win the radio galaxy ‘length competition’, it is probably necessary for Alcyoneus to have all important positive factors in its favor, we reasoned.

“In the discovery paper, we eliminate three simple hypotheses: that a very massive supermassive black hole, a high stellar mass, or a high radio power has led to Alcyoneus’ record length. So the big question still is how Alcyoneus got so big, and we think that studying this giant further could elucidate why radio galaxies in general become so large.”

By comparing the data with the formation of the largest galaxy ever discovered, scientists are expecting to gain more insights into the formation of galaxies.

Source: Martijn S.S.L. Oei et al, The discovery of a radio galaxy of at least 5 Mpc. arXiv:2202.05427v1 [astro-ph.GA],

The image at the top of the page is a joint radio-infrared view of Alcyoneus, a radio galaxy with a projected proper length of 5.0 megaparsecs. Researchers superimposed images from the LOFAR Two-meter Sky Survey (LoTSS), shown in orange, with images from the Wide-field Infrared Survey Explorer (WISE), shown in blue. ( Martijn Simon Soen Liong al.)

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via  Martijn Oei,  Reinout van Weeren  and Leiden University

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