Deciphering the large-scale environment of Radio Galaxies in the local universe: Where are they born? Where do they grow? Where do they die?

Abstract

The role played by the large-scale environment in the nuclear activity of radio galaxies (RGs) is still not completely understood. Accretion mode, jet power, and galaxy evolution are connected with their large-scale environment on scales from tens to hundreds of kilo parsecs. Here we present a detailed statistical analysis of the large-scale environment for two samples of RGs up to redshifts zsrc=0.15. The main advantages of our study over studies in the literature are the extremely homogeneous selection criteria of the catalogs adopted to perform our investigation.This is also coupled with the use of several clustering algorithms. We performed a direct search of galaxy-rich environments around RGs by using them as beacons. To perform this study we also developed a new method that does not appear to suffer from a strong zsrc dependence as other algorithms do. We conclude that, despite their radio morphological classification (FR I versus FR II) and/or their optical classification (high- or low-excitation radio galaxy (HERG or LERG)), RGs in the local universe tend to live in galaxy-rich large-scale environments that have similar characteristics and richness. We highlight that the fraction of FR I LERGs inhabiting galaxy-rich environments appears to be larger than that of FR II LERGs. We also found that five out of seven FR II HERGs,with zsrc„0.11, lie in groups/clusters of galaxies. However, we recognize that, despite the high level of completeness of our catalogs, when restricting to the local universe, the low number of HERGs (∼10% of the total FR IIs investigated) prevents us drawing a strong statistical conclusion about this source class.

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