Changing Ionization Conditions in SDSS Galaxies with AGN as a Function
  of Environment from Pairs to Clusters
  We study how AGN activity changes across environments from galaxy pairs to
clusters using $143\, 843$ galaxies with $z<0.2$ from the Sloan Digital Sky
Survey (SDSS). Using a refined technique, we apply a continuous measure of AGN
activity, characteristic of the ionization state of the narrow-line emitting
gas. Changes in key emission-line ratios ([NII]$\lambda6548$/H$\alpha$,
[OIII]$\lambda5007$/H$\beta$) between different samples allow us to disentangle
different environmental effects while removing contamination. We confirm that
galaxy interactions enhance AGN activity. However, conditions in the central
regions of clusters are inhospitable for AGN activity even if galaxies are in
pairs. These results can be explained through models of gas dynamics in which
pair interactions stimulate the transfer of gas to the nucleus and clusters
suppress gas availability for accretion onto the central black hole.
