Gas-Poor Hosts and Gas-Rich Companions of $z\approx$3.5 Radio Active Galactic Nuclei: ALMA Insights into Jet Triggering and Feedback

TL;DR

This study uses ALMA observations to reveal that $z oughly 3.5$ radio AGN hosts are gas-poor but have gas-rich companions, suggesting mergers or feedback processes influence galaxy evolution and AGN activity.

Contribution

It provides new high-resolution ALMA data showing the spatial distribution of cold gas around high-redshift radio AGN and their companions, highlighting the role of mergers and feedback.

Findings

Most cold gas is not in the host galaxies.

Hosts are gas-poor with nearby gas-rich companions.

Companions may be stripped clouds from mergers or affected by AGN feedback.

Abstract

Cold gaseous systems play important roles in galaxy evolution by possibly providing fuel to ignite active galactic nuclei (AGN) activity and star-formation. In this work, we analyze [CII]$158\rm \mu m$ and continuum observations from ALMA for a sample of four radio AGN at $z \approx 3.5$, focusing on eight associated companion cloud systems discovered within projected distances of tens of kiloparsecs or less. The spatial distribution of these companions indicates that the majority of cold gas is not located at the AGN position, i.e., not in their host galaxies. With the assistance of [CII] at $0.2"$ resolution, we further confirm the gas-poor nature of the hosts by re-analyzing archival [CI] (a tracer of H$_{2}$) at $\sim2"$ resolution. Our sample has [CII] luminosities in a range of $2.8\times10^{8}<L_{\rm [CII]}/L_{\odot}<4.2\times10^{9}$. The $L_{\rm [CII]}/L_{\rm IR}$ ratio, $\sim 9.4\times10^{-4}$, is consistent with sources discussed in the literature. Our findings show the gas-poor radio AGN hosts have nearby gas-rich companions. We propose that these companions may be stripped clouds resulting from merger processes, which could be a trigger of radio-loud AGN. They may also be a signature of negative AGN feedback (e.g., shock heating) on these infalling companions and on the host galaxy. In general, our analysis shows that powerful AGN at and before Cosmic Noon are impacting and being impacted by cold gaseous clouds in their circumgalactic or protointracluster media.