Alcyoneus (galaxy)

Lore & Background

Alcyoneus was first reported in a paper published in February 2022 by Martijn Oei and colleagues, based on results from the Low-Frequency Array (LOFAR) Two-metre Sky Survey (LoTSS). The object was observed as a bright, three-component radio structure visible at multiple spatial resolutions. The two outer components were separated by a distance similar to the smaller, elongated radio structure, indicating they were possible radio lobes. Radio-optical overlays confirmed that both lobes originated from the same source, not from separate galaxies.

The host galaxy, SDSS J081421.68+522410.0, is a standalone galaxy with an isophotal diameter of about 242,700 light-years (74.40 kpc). It does not host a quasar and is relatively quiescent, with a star formation rate of only 0.016 solar masses per year. The galaxy's stellar mass is 240 billion solar masses, and its central supermassive black hole has an estimated mass of 390±170 million solar masses—both typical for elliptical galaxies but lower than for similar galaxies generating giant radio sources.

It is currently unknown how Alcyoneus's radio emissions grew so large. One explanation proposes that the cosmic web environment around the galaxy might be less dense than that of other giant radio galaxies, leading to lower resistance to growth. Compared to other known giant radio galaxies, Alcyoneus does not appear to have a particularly massive stellar population, central black hole, or powerful jet streams.

Reader's Guide

Alcyoneus is significant as a giant radio galaxy that, at the time of its discovery, held the record for the largest known structure of galactic origin, with radio lobes spanning 5 megaparsecs. Its discovery in LOFAR data as part of the LoTSS survey contributed to the identification of over 8,000 new giant radio galaxies, highlighting the power of low-frequency radio surveys in revealing large-scale structures. The galaxy's relatively ordinary host properties—modest stellar mass, black hole mass, and radio luminosity—challenge existing models of how such enormous radio structures can form. The fact that Alcyoneus has since been superseded by even larger radio galaxies (SDSS J081956.41+323537.6 and Porphyrion) underscores the rapid progress in this field. Its legacy lies in prompting further investigation into the role of the cosmic web environment in enabling the growth of giant radio galaxies, and in demonstrating that extreme radio structures can arise from seemingly unremarkable galaxies.

Did You Know?

• Alcyoneus was named after the giant Alcyoneus from Greek mythology.
• Its radio lobes span 5 megaparsecs (16 million light-years), making it the second-largest radio galaxy known as of 2024.
• The host galaxy has a star formation rate of only 0.016 solar masses per year.
• The nearest cluster to Alcyoneus is located 11 million light-years away.