XXL Survey XXI. The environment and clustering of X-ray AGN in the
  XXL-South field
  This work is part of a series of studies focusing on the environment and the
properties of the X-ray selected active galactic nuclei (AGN) population from
the XXL survey. The present survey, given its large area, continuity, extensive
multiwavelength coverage, and large-scale structure information, is ideal for
this kind of study. Here, we focus on the XXL-South (XXL-S) field. Our main aim
is to study the environment of the various types of X-ray selected AGN and
investigate its possible role in AGN triggering and evolution. We studied the
large-scale (>1 Mpc) environment up to redshift z=1 using the nearest neighbour
distance method to compare various pairs of AGN types. We also investigated the
small-scale environment (<0.4 Mpc) by calculating the local overdensities of
optical galaxies. In addition, we built a catalogue of AGN concentrations with
two or more members using the hierarchical clustering method and we correlated
them with the X-ray galaxy clusters detected in the XXL survey. It is found
that radio detected X-ray sources are more obscured than non-radio ones, though
not all radio sources are obscured AGN. We did not find any significant
differences in the large-scale clustering between luminous and faint X-ray AGN,
or between obscured and unobscured ones, or between radio and non-radio
sources. At local scales (<0.4 Mpc), AGN typically reside in overdense regions,
compared to non-AGN; however, no differences were found between the various
types of AGN. A majority of AGN concentrations with two or more members are
found in the neighbourhood of X-ray galaxy clusters within <25-45 Mpc. Our
results suggest that X-ray AGN are typically located in supercluster filaments,
but they are also found in over- and underdense regions.
