The Infrared Medium-Deep Survey II: How to Trigger Radio-AGN? Hints from Their Environments

TL;DR

This study investigates the environments of radio-AGN using infrared and radio data, finding that most are in typical environments, but a subset exists in much denser regions, suggesting different triggering mechanisms.

Contribution

It provides new insights into the environmental conditions of radio-AGN and challenges the idea that galaxy mergers are the primary trigger for their activity.

Findings

Most radio-AGN are in environments similar to control galaxies.

A sub-population of radio-AGN exists in environments up to 100 times denser.

Radio-AGN in less dense environments show higher radio-loudness and star formation efficiencies.

Abstract

Activity at the centers of galaxies, during which the central supermassive black hole is accreting material, is nowadays accepted to be rather ubiquitous and most probably a phase of every galaxy's evolution. It has been suggested that galactic mergers and interactions may be the culprits behind the triggering of nuclear activity. We use near-infrared data from the new Infrared Medium-Deep Survey (IMS) and the Deep eXtragalactic Survey (DXS) of the VIMOS-SA22 field and radio data at 1.4 GHz from the FIRST survey and a deep VLA survey to study the environments of radio-AGN over an area of ~25 sq. degrees and down to a radio flux limit of 0.1 mJy and a J-band magnitude of 23 mag AB. Radio-AGN are predominantly found in environments similar to those of control galaxies at similar redshift, J-band magnitude, and U-R rest-frame absolute color. However, a sub-population of radio-AGN is found in environments up to 100 times denser than their control sources. We thus preclude merging as the dominant triggering mechanism of radio-AGN. Through the fitting of the broadband spectral energy distribution of radio-AGN in the least and most dense environments, we find that those in the least dense environments show higher radio-loudness, higher star formation efficiencies, and higher accretion rates, typical of the so-called high-excitation radio-AGN. These differences tend to disappear at z>1. We interpret our results in terms of a different triggering mechanism for these sources that is driven by mass-loss through winds of young stars created during the observed ongoing star formation.