The discovery of a radio galaxy of at least 5 Mpc

TL;DR

This paper reports the discovery of Alcyoneus, the largest known radio galaxy at nearly 5 Mpc, providing insights into galaxy growth and intergalactic medium interactions.

Contribution

It presents the identification and characterization of Alcyoneus, the largest radio galaxy, and discusses its implications for understanding galaxy evolution and cosmic web interactions.

Findings

Alcyoneus has a projected length of approximately 5 Mpc.

Its low-frequency luminosity and black hole mass are comparable to typical giant radio galaxies.

It resides in a cosmic filament with extremely low lobe pressure, ideal for probing intergalactic medium.

Abstract

We discover what is in projection the largest known structure of galactic origin: a giant radio galaxy with a projected proper length of $4.99 \pm 0.04\ \mathrm{Mpc}$. The source, named Alcyoneus, was first identified in low-resolution LOFAR Two-metre Sky Survey images from which angularly compact sources had been removed. Being an extreme example in its class, Alcyoneus could shed light on the main mechanisms that drive radio galaxy growth. We find that - beyond geometry - Alcyoneus and its host galaxy appear suspiciously ordinary: the total low-frequency luminosity density, stellar mass and supermassive black hole mass are all lower than, though similar to, those of the medial giant radio galaxy (percentiles $45 \pm 3\%$, $25 \pm 9 \%$ and $23 \pm 11 \%$, respectively). The source resides in a filament of the Cosmic Web, with which it might have significant thermodynamic interaction. At $5 \cdot 10^{-16}\ \mathrm{Pa}$, the pressures in the lobes are the lowest hitherto found, and Alcyoneus therefore represents one of the most promising radio galaxies yet to probe the warm-hot intergalactic medium.