2019A&A...624A..91K

Context. Giant radio galaxies (with projected linear size of radio structure larger than 0.7Mpc) are very rare and unusual objects. Only ∼5% of extended radio sources reach such sizes. Understanding of the processes responsible for their large sizes is crucial to further our knowledge about the radio source's evolution.
Aims. We check the hypothesis that giants become extremely large due to the specific history of their host galaxy formation, as well as in the context of the cluster or group of galaxies where they evolve. Therefore we study the star formation histories in their host galaxies and in galaxies located in their neighbourhood.
Methods. We studied 41 giant-size radio galaxies as well as galaxies located within a radius of 5Mpc around giants to verify whether the external conditions of the intergalactic medium somehow influence the internal evolution of galaxies in the group/cluster. We compared the results with a control sample of smaller-sized Fanaroff-Riley type II radio galaxies and their neighbouring galaxies. We fit stellar continua in all galaxy spectra using the spectral synthesis code STARLIGHT and provide statistical analysis of the results.
Results. We find that hosts of giant radio galaxies have a larger amount of intermediate age stellar populations compared with smaller-sized FRII radio sources. The same result is also visible when we compare neighbouring galaxies located up to 1.5Mpc around giants and FRIIs. This may be evidence that star formation in groups with giants was triggered due to global processes occurring in the ambient intergalactic medium. These processes may also contribute to mechanisms responsible for the extremely large sizes of giants.