NoSOCS in SDSS. VI. The Environmental Dependence of AGN in Clusters and
  Field in the Local Universe
  We investigated the variation in the fraction of optical active galactic
nuclei (AGN) hosts with stellar mass, as well as their local and global
environments. Our sample is composed of cluster members and field galaxies at
$z \le 0.1$ and we consider only strong AGN. We find a strong variation in the
AGN fraction ($F_{AGN}$) with stellar mass. The field population comprises a
higher AGN fraction compared to the global cluster population, especially for
objects with log $M_* > 10.6$. Hence, we restricted our analysis to more
massive objects. We detected a smooth variation in the $F_{AGN}$ with local
stellar mass density for cluster objects, reaching a plateau in the field
environment. As a function of clustercentric distance we verify that $F_{AGN}$
is roughly constant for R $> $ R$_{200}$, but show a steep decline inwards. We
have also verified the dependence of the AGN population on cluster velocity
dispersion, finding a constant behavior for low mass systems ($\sigma_P
\lesssim 650-700$ km s$^{-1}$). However, there is a strong decline in $F_{AGN}$
for higher mass clusters ($>$ 700 km s$^{-1}$). When comparing the $F_{AGN}$ in
clusters with or without substructure we only find different results for
objects at large radii (R $> $ R$_{200}$), in the sense that clusters with
substructure present some excess in the AGN fraction. Finally, we have found
that the phase-space distribution of AGN cluster members is significantly
different than other populations. Due to the environmental dependence of
$F_{AGN}$ and their phase-space distribution we interpret AGN to be the result
of galaxy interactions, favored in environments where the relative velocities
are low, typical of the field, low mass groups or cluster outskirts.
