The environmental properties of radio-emitting AGN

Abstract

We study the environmental properties of z ≲ 1.2 radio-selected active galactic nuclei (AGNs) belonging to the ~2 deg2 of the COSMOS field, finding that about 20 per cent of them appear within overdense structures. AGNs with P1.4 GHz > 1023.5 WHz-1 sr-1 are twice more likely to be found in clusters with respect to fainter sources (~38 per cent vs ~15 per cent), just as radio-selected AGNs with stellar masses M* > 1011M⊙ are twice more likely to be found in overdense environments with respect to objects of lower mass (~24 per cent vs ~11 per cent). Comparisons with galaxy samples further suggest that radio-selected AGN of large stellar mass tend to avoid underdense environments more than normal galaxies with the same stellar content. Stellar masses also seem to determine the location of radio-active AGN within clusters: ~100 per cent of the sources found as satellite galaxies have M* < 1011.3M⊙, while ~100 per cent of the AGNs coinciding with a cluster central galaxy have M* > 1011M⊙. No different location within the cluster is instead observed for AGN of various radio luminosities. Radio AGN, which also emit in the Mid-Infrared show a marked preference to be found as isolated galaxies (~70 per cent) at variance with those also active in the X-ray that all seem to reside within overdensities. What emerges from our work is a scenario whereby physical processes on sub-pc and kpc scales (e.g. emission, respectively, related to the AGN and to star formation) are strongly interconnected with the large-scale environment of the AGN itself.