Direct effects of the environment on AGN triggering in SDSS spiral
  galaxies: merger-AGN connection
  We examine whether galaxy environments directly affect triggering nuclear
activity in Sloan Digital Sky Survey (SDSS) local spiral galaxies using a
volume-limited sample with the $r$-band absolute magnitude $M_{r} < -19.0$ and
$0.02 < z < 0.055$ selected from the SDSS Data Release 7. To avoid
incompleteness of the central velocity dispersion $\sigma$ of the
volume-limited sample and to fix the black hole mass affecting AGN activity, we
limit the sample to a narrow $\sigma$ range of $130$ km s$^{-1}<\sigma<200$ km
s$^{-1}$. We define a variety of environments as a combination of neighbour
interactions and local density on a galaxy. After the central star formation
rate (which is closely related to AGN activity level) is additionally
restricted, the direct impact of the environment is unveiled. In the outskirts
of rich clusters, red spiral galaxies show a significant excess of the AGN
fraction despite the lack of central gas. We argue that they have been
pre-processed before entering the rich clusters, and due to mergers or strong
encounters in the in-fall region, their remaining gases efficiently lose
angular momentum. We investigate an environment in which many star-forming
galaxies coexist with a few starburst-AGN composite hosts having the highest
[OIII] luminosity. We claim that they are a gas-rich merger product in groups
or are group galaxies in-falling into clusters, indicating that many AGN
signatures may be obscured following the merger events.
