AGNs in Small Galaxy Systems: comparing the main properties of active
  objects in pairs, triplets and groups
  We perform a comparative study of AGNs in pairs, triplets and groups. To this
end we use the Duplancic et al. catalogue of small galaxy systems and consider
BPT and WHAN diagnostic diagrams to select optical AGNs. Also we identify
mid-IR AGNs by using WISE data. We performed a comparison between the different
AGN classification methodologies and study the AGN fraction in pairs, triplets,
and groups with four to six members. We also analyse the main properties of
Optical and mid-IR AGN hosts and the influence of environment on the active
nuclei phenomena in these small galaxy systems. Our results show that,
regardless the specifically adopted classification scheme, the fraction of AGN
in pairs and triplets is always higher than the corresponding fraction in
groups. Moreover, the fraction of powerful AGNs in pair and triplets is about
twice the fraction of regular AGNs. We also find a remarkable difference
between Optical and mid-IR AGNs in groups, where host galaxies of WISE AGNs are
less massive and concentrated, with young stellar populations and blue colours.
Also all WISE AGNs in groups have a very close companions and reside in an
intermediate global density environment. Galaxy triplets show a larger AGN
fraction for galaxies with a close nearest neighbours, while pairs present a
nearly constant AGN fraction regardless the distance to the nearest companion.
Our studies highlight the important role of interactions, besides the global
environment dependence, in the activation of the AGN phenomenon in small galaxy
systems.
