Mergers as triggers for nuclear activity: A near-IR study of the close
  environment of AGN in the VISTA-VIDEO survey
  There is an ongoing debate concerning the driver of nuclear activity in
galaxies, with AGN either being triggered by major or minor galactic mergers
or, alternatively, through secular processes like cooling gas accretion and/or
formation of bars. We investigate the close environment of active galaxies
selected in the X-ray, the radio, and the mid-IR. We utilise the first data
release of the new near-IR VISTA Deep Extragalactic Observations (VIDEO) survey
of the XMM-Large Scale Structure (LSS) field. We use two measures of
environment density, namely counts within a given aperture and a finite
redshift slice (pseudo-3D density) and closest neighbour density measures
${\Sigma}_{2}$ and ${\Sigma}_{5}$. We select both AGN and control samples,
matching them in redshift and apparent Ks-band magnitude. We find that AGN are
found in a range of environments, with a subset of the AGN samples residing in
over-dense environments. Seyfert-like X-ray AGN and flat-spectrum radio-AGN are
found to inhabit significantly over-dense environments compared to their
control sample. The relation between over-densities and AGN luminosity does not
however reveal any positive correlation. Given the absence of an environment
density-AGN luminosity relation,we find no support for a scheme where high
luminosity AGN are preferentially triggered by mergers. On the contrary, we
find that AGN likely trace over dense environments at high redshift due to the
fact that they inhabit the most massive galaxies, rather than being an AGN.
