An ordinary galaxy blowing two lobes of radio plasma across 16 million light-years
Artist's impression · drag to rotate
Lengthat least 5 megaparsecs4.99 ± 0.04 Mpc on the sky, about 16 million light-years[1]
Size on the sky20.8 arcminutesabout as wide as the full Moon, if you could see it[2]
Distanceabout 3 billion light-yearsredshift 0.247; Wikipedia gives 3.5 billion[2]
Host galaxyabout 74 kiloparsecs acrossan elliptical galaxy in Lynx[5]
Central black holeabout 400 million Sunsabout a quarter of the usual for giant radio galaxies[1]
Age of the lobesat least 1.7 to 2.1 billion yearsthe minimum time to inflate them[1]
Seen withLOFAR, 144 MHzfound by reprocessing survey images[1]
Record heldFebruary 2022 to September 2024overtaken by Porphyrion, about 7 megaparsecs[4]
About the image. Drawn as a small galaxy with two thin jets and two vast, faint lobes of glowing blue and violet gas. The lobes are seen only at radio wavelengths and would be invisible to the eye, so every colour here is invented; their size and the north–south layout follow LOFAR's radio maps.
Alcyoneus is a galaxy about 3 billion light-years away that has blown two lobes of radio-glowing gas out into space. From end to end they stretch at least 5 megaparsecs, about 16 million light-years. In 2022 that made it the largest radio galaxy known. On the sky it spans 20.8 arcminutes, about as wide as the full Moon, and yet nobody could ever see it with their eyes: the lobes give off only radio waves.
Found in old data
Oei and his colleagues at Leiden University were searching images from the LOFAR radio telescope for giant radio galaxies. Faint, spread-out plumes are easy to miss, so they reprocessed the survey images to bring out such structures. Two lobes, one north and one south of a galaxy in the constellation Lynx, turned out to belong to the same source. The name was chosen by Aivin Gast, an undergraduate at Somerville College, Oxford, and a co-author of the paper. In Greek myth Alcyoneus was a giant who fought Heracles.
An ordinary engine
The lobes are made of charged particles and magnetic fields, pushed out by jets from the galaxy’s central black hole over at least 1.7 to 2.1 billion years. The team expected a monster at the centre. They found the opposite. Compared with the typical giant radio galaxy, the black hole of Alcyoneus has only about a quarter of the mass, its galaxy about a quarter of the stars, and its lobes less than half the radio power. Nothing about the engine explains the size. The leading idea is that the galaxy sits in a thin part of the cosmic web, where there is little gas to slow the lobes down, but that is not proven.
Not the largest galaxy
Headlines in 2022 called Alcyoneus the largest galaxy ever found. That is not quite right. The galaxy itself, its stars, is about 74 kiloparsecs across. The record is for the radio lobes. By the extent of its stars, the largest known galaxy is IC 1101.
Its own record lasted two and a half years. In September 2024 the same team announced Porphyrion, a pair of black-hole jets about 7 megaparsecs, or 23 million light-years, long. It lies 7.5 billion light-years away, and its jets formed when the universe was 6.3 billion years old.
Gallery
A small bright point in the centre with two thin blue jets shooting out in opposite directions through faint bluish clouds of gas, an impression of the giant radio galaxy Alcyoneus. AI-generated painting.A small pink-white galaxy at the lower left sending a straight blue beam across dark space into a huge, billowing violet cloud, an impression of one of the radio lobes of Alcyoneus. AI-generated painting.Alcyoneus, 3D artist's impression, rendered from the interactive model.Alcyoneus, 3D artist's impression, from another angle, rendered from the interactive model.Alcyoneus, 3D artist's impression, close-up, rendered from the interactive model.
Timeline
2018Oei's team begins searching LOFAR images for giant radio galaxies.
2022Alcyoneus is announced in February as the largest radio galaxy known, at least 5 megaparsecs long.
2024Porphyrion, about 7 megaparsecs long, takes the record in September.
Questions people ask
Is Alcyoneus the largest galaxy?
Not by its stars. Its radio lobes span about 16 million light-years, but the galaxy itself is about 74 kiloparsecs across. By the extent of its stars, the largest known galaxy is IC 1101.
How big is Alcyoneus?
At least 5 megaparsecs, about 16 million light-years, from the tip of one radio lobe to the other.
What is bigger than Alcyoneus?
Porphyrion, announced in 2024: a pair of black-hole jets about 23 million light-years long, 7.5 billion light-years away.