Why z > 1 radio-loud galaxies are commonly located in proto-clusters

TL;DR

Radio-loud active galactic nuclei at high redshift are predominantly found in dense proto-cluster environments, and this study suggests that such environments facilitate the formation of radio jets, impacting galaxy evolution.

Contribution

This paper demonstrates that dense environments at z > 1.3 promote radio jet formation, and quantifies the impact of radio-loud feedback on proto-cluster gas heating.

Findings

RLAGN environments are significantly denser than radio-quiet galaxies.

Less than 50% of massive galaxies host RLAGN at this epoch.

Radio-loud feedback heats the intracluster medium, affecting star formation.

Abstract

Distant powerful radio-loud active galactic nuclei (RLAGN) tend to reside in dense environments and are commonly found in proto-clusters at z > 1.3. We examine whether this occurs because RLAGN are hosted by massive galaxies, which preferentially reside in rich environments. We compare the environments of powerful RLAGN at 1.3 < z < 3.2 from the CARLA survey to a sample of radio-quiet galaxies matched in mass and redshift. We find the environments of RLAGN are significantly denser than those of radio-quiet galaxies, implying that not more than 50% of massive galaxies in this epoch can host powerful radio-loud jets. This is not an observational selection effect as we find no evidence to suggest it is easier to observe the radio emission when the galaxy resides in a dense environment. We therefore suggest that the dense Mpc-scale environment fosters the formation of a radio-jet from an AGN. We show that the number density of potential RLAGN host galaxies is consistent with every > 10^14 solar mass cluster having experienced powerful radio-loud feedback of duration ~60 Myr during 1.3 < z < 3.2. This feedback could heat the intracluster medium to the extent of 0.5-1 keV per gas particle, which could limit the amount of gas available for further star formation in the proto-cluster galaxies.