Alcyoneus is a low-excitation, Fanaroff–Riley class II radio galaxy located 3.5 billion light-years (1.1 gigaparsecs) from Earth, with host galaxy SDSS J081421.68+522410.0. It is located in the constellation Lynx and it was discovered in Low-Frequency Array (LOFAR) data by a team of astronomers led by Martijn Oei. As of 2024, it has the second-largest extent of radio structure of any radio galaxy identified, with lobed structures spanning 5 megaparsecs (16 million light-years) across, described by its discoverers at the time as the "largest known structure of galactic origin." It has since been superseded by two other radio galaxies, SDSS J081956.41+323537.6 and Porphyrion, with lobed structures of 5.07 megaparsecs (17 million light-years) and 7 megaparsecs (23 million light-years).
Aside from the size of its radio emissions, the central galaxy is otherwise of ordinary radio luminosity, stellar mass, and supermassive black hole mass. It is a standalone galaxy with an isophotal diameter at 25.0 r-mag/arcsec of about 242,700 light-years (74.40 kpc), with the nearest cluster located 11 million light-years away from it. The galaxy was named after the giant Alcyoneus from Greek mythology.
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