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