Astronomers have discovered the largest radio galaxy ever, 100 times larger than the Milky Way

 

Alcyoneus: The Largest Radio Galaxy Revealed in Unprecedented Detail

Astronomers have made an extraordinary discovery, identifying the largest radio galaxy on record: Alcyoneus, which stretches over 16 million light-years across space. Situated about 3 billion light-years from Earth, Alcyoneus was detected by scientists using data from the Low-Frequency Array (LOFAR) network of radio telescopes across Europe. Named after the mythological Greek giant Alcyoneus, this galactic behemoth represents the largest known structure originating from a galaxy, measuring more than 100 times the diameter of the Milky Way.

Discovery and Structure of Alcyoneus

Radio galaxies are unique in that they host supermassive black holes at their centers. When matter is pulled into these black holes, they emit powerful streams of energy in the form of radio jets. In Alcyoneus’ case, the galaxy’s central black hole emits two enormous plasma plumes, which extend outwards to a length of 16 million light-years, creating the largest radio galaxy structure ever recorded. The jets’ material heats up to form plasma, emitting radio waves that LOFAR can detect. Previously, astronomers had missed these faint signals, but by reprocessing existing images, they were able to distinguish the faint yet massive structure of Alcyoneus.

These plumes are particularly noteworthy because they represent more than just an energy output from the galaxy’s core. They contain material essential for the formation of new stars, which means Alcyoneus is not only a colossal structure but also a site of potential star-forming material that spreads over vast cosmic distances.

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. (Image credit: Martijn Oei et al.)

The Mystery of Alcyoneus’ Size and Cosmic Web Connections

What makes Alcyoneus so remarkable is not just its scale but also its relative normalcy. Alcyoneus’s supermassive black hole, jet strength, and stardust content all appear average compared to other radio galaxies, which raises questions about how this particular galaxy reached such enormous proportions. This puzzling factor is leading scientists to consider other influences, such as the impact of Alcyoneus’ intergalactic environment.

Researchers are also considering whether Alcyoneus might provide insights into the “cosmic web,” a theoretical framework describing how galaxies are connected across vast distances by clusters and filaments. Alcyoneus’ immense plumes might be influenced by forces in this cosmic web or by intergalactic medium interactions. By investigating the galaxy’s surroundings, astronomers hope to uncover factors that contributed to Alcyoneus’s unusual growth.

Through continued study, Alcyoneus may shed new light on how large radio galaxies form and expand. As data from LOFAR and other telescopes improve, scientists anticipate deeper insights into how cosmic structures are shaped and influenced by the universe’s unseen forces.

The findings have been accepted for publication in the journal Astronomy & Astrophysics.