Properties of giant radio galaxies larger than 3 Mpc

Context. Giant radio galaxies (GRGs) are radio galaxies with radio emission exceeding 0.7 Mpc in physical size. Recently, the GRG sample has grown large enough to study the extreme end of the GRG size distribution. Aims. We examine the properties of GRGs with largest linear sizes greater than 3 Mpc to shed light on their nature and origin. Methods. We selected, corroborated, and, where necessary, revised the largest GRGs from the literature. We added to these the GRGs identified in our own search of modern radio survey images, combined with optical surveys and catalogues of spectroscopic and photometric redshifts, to determine their projected linear radio size. We studied their radio power–size relation, lobe asymmetry, association with galaxy clusters, and bending angles. Results. We present an unprecedented sample of 142 GRGs larger than 3 Mpc, of which 69 are newly identified in this work. The sample includes six GRGs with projected linear sizes exceeding 5 Mpc and reaching up to 6.6 Mpc. We find that GRGs larger than 3 Mpc are distributed in redshift and radio luminosity indistinguishable from those of smaller GRGs. The same applies to the fraction of quasars among their hosts. At most, a single GRGs larger than 3 Mpc can be classified as a clear Fanaroff-Riley (FR) type I source, and only six per cent deviate from a clear FR II radio morphology. One quarter of our GRGs show very diffuse lobes typical for remnant radio galaxies, and only 59 per cent show indications of hotspots in at least one lobe, with 38 per cent featuring a hotspot in both lobes. As in the case of smaller radio galaxies, the shorter lobe is most often also the brighter one. We find tentative evidence that the bending angle decreases with GRG size, but no trend with redshift is detected. The fraction of GRGs > 3 Mpc associated with catalogued clusters of galaxies is around 15 per cent, and thus indistinguishable from that of smaller GRGs. The bending angles of GRGs > 3 Mpc in known clusters are larger than those of GRGs not associated with clusters.