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  • Gursheen Kaur Obhan


Astronomers discovered a truly enormous galaxy earlier this year. The distance to Alcyoneus, a massive radio galaxy that stretches 5 megaparsecs into space, was about 3 billion light-years. At a length of 16.3 million light-years, it is the longest structure of known galactic origin.

After acquiring data from the Low Frequency Array (LOFAR) Two-metre Sky Survey (LoTSS), an interferometric radio survey of the Northern Sky, Martijn Oei and colleagues presented an article in February 2022 in which they first described Alcyoneus. The item was initially identified as a brilliant, three-component radio structure that was visible on at least four different LoTSS spatial resolutions. Similar to the smaller, elongated radio structure, the two outer components of the radio structure are spaced apart, suggesting that they could be radio lobes. Further confirmations made with radio-optical overlays rule out the hypothesis that the two radio lobes are distinct entities formed by different galaxies and support the idea that they came from the same source.

The structure was given the name "Alcyoneus" by the first study, which was inspired by one of Hercules' foes and the enormous son of Uranus and the Titan Gaia in Greek mythology.

Giant radio galaxies, such as Alcyoneus, are a distinct class of objects distinguished by the presence of relativistic jets propelled by the supermassive black hole at the centre of the galaxy. According to spectral imaging from the Sloan Digital Sky Survey's 12th data release (SDSS DR12), Alcyoneus' host galaxy does not include a quasar (an extraordinarily bright active galactic core), and instead has a star creation rate of only 0.016 solar masses per year.As a result, Alcyoneus is categorised as a low excitation radio source because the central galaxy's jet, rather than radiation from the active galactic centre, relativistically processes most of the energy that it emits.

Alcyoneus' primary host galaxy has a star mass of 240 billion sun masses, and its supermassive black hole is thought to have a mass of 390–170 million solar masses, which is common for elliptical galaxies but significantly less than that of other galaxies that host other large radio emitters.

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