Aims. To deduce the main mechanisms that drive a phenomenon, it is usually instructive to study extreme examples. If there exist host galaxy characteristics that are an important cause for GRG growth, then the hosts of the largest GRGs are likely to possess them. Similarly, if there exist particular large-scale environments that are highly conducive to GRG growth, then the largest GRGs are likely to reside in them. For these reasons, we aim to perform a case study of the largest GRG available.
Methods. We reprocessed the LOFAR Two-metre Sky Survey (LoTSS) DR2 by subtracting compact sources and performing multi- scale CLEAN deconvolution at $60^{\prime\prime}$ and $90^{\prime\prime}$ resolution. The resulting images constitute the most sensitive survey yet for radio galaxy lobes, whose diffuse nature and steep synchrotron spectra have allowed them to evade previous detection attempts at higher resolution and shorter wavelengths. We visually searched these images for GRGs.
Results. We discover Alcyoneus, a low-excitation radio galaxy with a projected proper length $l_p = 4.99 \pm 0.04\ \rm Mpc$. Its jets and lobes are all four detected at very high significance, and the SDSS-based identification of the host, at spectroscopic redshift $z_{spec} = 0.24674 \pm 6\times 10^{-5}$, is unambiguous. The total luminosity density at $ν = 144\ \rm MHz$ is $L_ν = 8\pm 1\times 10^{25}\ \rm W\ Hz^{-1}$, which is below-average, though near-median (percentile $45 \pm 3\text{%}$), for GRGs. The host is an elliptical galaxy with a stellar mass $M_\star = 2.4 \pm 0.4\times 10^{11}\ M_\odot$ and a supermassive black hole mass $M_\bullet =4\pm 2\times 10^8 M_\odot$, both of which tend towards the lower end of their respective GRG distributions (percentiles $25\pm 9\text{%}$ and $23\pm 11\text{%}$). The host resides in a filament of the Cosmic Web. Through a new Bayesian model for radio galaxy lobes in three dimensions, we estimate the pressures in the $\rm Mpc^3$-scale northern and southern lobe to be $P_{min,1} = 4.8 \pm 0.3\times 10^{-16}\ \rm Pa$ and $P_{min,2} = 4.9 \pm 0.6\times 10^{-16}\ \rm Pa$, respectively. The corresponding magnetic field strengths are $B_{min,1} = 46\pm 1\ \rm pT$ and $B_{min,2} = 46\pm 3\ \rm pT$.
Methods. We reprocessed the LOFAR Two-metre Sky Survey (LoTSS) DR2 by subtracting compact sources and performing multi- scale CLEAN deconvolution at $60^{\prime\prime}$ and $90^{\prime\prime}$ resolution. The resulting images constitute the most sensitive survey yet for radio galaxy lobes, whose diffuse nature and steep synchrotron spectra have allowed them to evade previous detection attempts at higher resolution and shorter wavelengths. We visually searched these images for GRGs.
Results. We discover Alcyoneus, a low-excitation radio galaxy with a projected proper length $l_p = 4.99 \pm 0.04\ \rm Mpc$. Its jets and lobes are all four detected at very high significance, and the SDSS-based identification of the host, at spectroscopic redshift $z_{spec} = 0.24674 \pm 6\times 10^{-5}$, is unambiguous. The total luminosity density at $ν = 144\ \rm MHz$ is $L_ν = 8\pm 1\times 10^{25}\ \rm W\ Hz^{-1}$, which is below-average, though near-median (percentile $45 \pm 3\text{%}$), for GRGs. The host is an elliptical galaxy with a stellar mass $M_\star = 2.4 \pm 0.4\times 10^{11}\ M_\odot$ and a supermassive black hole mass $M_\bullet =4\pm 2\times 10^8 M_\odot$, both of which tend towards the lower end of their respective GRG distributions (percentiles $25\pm 9\text{%}$ and $23\pm 11\text{%}$). The host resides in a filament of the Cosmic Web. Through a new Bayesian model for radio galaxy lobes in three dimensions, we estimate the pressures in the $\rm Mpc^3$-scale northern and southern lobe to be $P_{min,1} = 4.8 \pm 0.3\times 10^{-16}\ \rm Pa$ and $P_{min,2} = 4.9 \pm 0.6\times 10^{-16}\ \rm Pa$, respectively. The corresponding magnetic field strengths are $B_{min,1} = 46\pm 1\ \rm pT$ and $B_{min,2} = 46\pm 3\ \rm pT$.
Conclusions. We have discovered what is in projection the largest known structure made by a single galaxy — a GRG with a projected proper length $l_p = 4.99 \pm 0.04\ \rm Mpc$. The true proper length is at least $l_{min} = 5.04 \pm 0.05\ \rm Mpc$. Beyond geometry, Alcyoneus and its host are 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 GRG. Thus, very massive galaxies or central black holes are not necessary to grow large giants, and, if the observed state is representative of the source over its lifetime, neither is high radio power. A low- density environment remains a possible explanation. The source resides in a filament of the Cosmic Web, with which it might have significant thermodynamic interaction. The pressures in the lobes are the lowest hitherto found, and Alcyoneus therefore represents the most promising radio galaxy yet to probe the warm–hot intergalactic medium.
