The discovery of a radio galaxy of at least 5 Mpc

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.

• The paper identifies Alcyoneus as a radio galaxy with a projected proper length of nearly 5 Mpc, marking it as one of the largest observed.
• The study employs LOFAR LoTSS DR2 data and Bayesian models to analyze radio lobe pressures and magnetic field strengths, emphasizing environmental influences.
• The analysis reveals that ordinary elliptical hosts with modest black hole masses can form giant radio galaxies, challenging traditional growth theories.

An Analysis of the Discovery of a 5 Mpc Radio Galaxy: Alcyoneus

The paper details the discovery of Alcyoneus, a radio galaxy noted for its considerable size, with a projected proper length of approximately 5 Mpc, potentially representing the largest such structure observed thus far. This study, grounded in data from the Low-frequency Array (LOFAR) Two-metre Sky Survey (LoTSS) Data Release 2 (DR2), aims to provide insight into the growth mechanisms of giant radio galaxies (GRGs) through the analysis of extreme examples.

Key Findings

• Size and Structure: Alcyoneus is identified as a low-excitation radio galaxy (LERG) located at a redshift of 0.24674. It exhibits a total projected proper length of 4.99 ± 0.04 Mpc with a minimum true proper length of at least 5.04 ± 0.05 Mpc.
• Host Galaxy Characteristics: The host galaxy of Alcyoneus is an elliptical galaxy with a stellar mass of around 2.4 ± 0.4 × 10^11 M_⊙ and a black hole mass of 4 ± 2 × 10^8 M_⊙, both of which are at the lower end of the distributions typical for GRGs. This indicates that neither exceptionally massive galaxies nor supermassive black holes are prerequisites for GRG growth.
• Radio and Environmental Properties: Alcyoneus is characterized by a total low-frequency luminosity density typical for GRGs, and not disproportionately high for its size. Notably, the galaxy resides in a filament of the Cosmic Web rather than a cluster, suggesting environmental density may influence its growth trajectory.
• Pressure and Energy Dynamics: Through a Bayesian model of radio galaxy lobes, the pressures within Alcyoneus' lobes are calculated to be around 4.8 × 10^-16 Pa, and the magnetic field strengths are approximately 46 pT. The study notes these pressures are compatible with the densest parts of the warm–hot intergalactic medium (WHIM), proposing Alcyoneus as a potential intergalactic barometer.

Implications and Future Directions

The implications of these findings are considerable for our understanding of radio galaxy formation and evolution. The identification of a giant radio galaxy with relatively ordinary host characteristics challenges the notion that exceptional intrinsic properties are necessary for developing such large-scale structures. This suggests that environmental factors, such as residing in a low-density filament, might significantly influence the size of radio galaxies.

Alcyoneus provides a striking example of how future research might harness extreme cases to test the limits of growth theories. Further investigations could explore the environmental conditions at play in locations away from clusters, which are often assumed to host GRGs. Moreover, the application of advanced statistical models to other radio galaxies might refine our understanding of the interplay between intrinsic properties and external influences.

The discovery of Alcyoneus encourages consideration of non-trivial growth mechanisms, perhaps involving dynamic interactions with the WHIM. Additionally, it underscores the potential of continued deep surveys like LOFAR to uncover even fainter examples of similar structures that remain undetected, providing data to test and develop models of radio galaxy evolution.

Overall, Alcyoneus stands as a noteworthy entry in the ongoing exploration of cosmic giants, setting a baseline for studies thoroughly examining connections between galaxy properties, environmental factors, and the extremes of radio galaxy size distribution.