The importance of galaxy interactions in triggering type II quasar
  activity
  We present deep Gemini GMOS-S optical broad-band images for a complete sample
of 20 SDSS selected type II quasars with redshifts 0.3 < z < 0.41 and [OIII]
emission line luminosities greater than 10^8.5 solar luminosities. We use these
images to determine the significance of galaxy interactions in triggering
nuclear activity, finding that 15 (75%) show evidence for interaction in the
form of tails, shells, double nuclei etc. The median surface brightness of the
features is 23.4 mag arcsec sq and the range is 20.9-24.7 mag arcsec sq.
  We find a similar rate of interaction in the type II quasars as in a
comparison sample of quiescent early-type galaxies at similar redshift (67 +/-
14%). However the surface brightness of the detected features is up to 2
magnitudes brighter for the type II quasars than for the quiescent early-types,
which have surface brightnesses in the range 22.1-26.1 mag arcsec sq and a
median surface brightness 24.3 mag arcsec sq. This may indicate that the
mergers witnessed in the comparison sample galaxies could have different
progenitors, or we may be viewing the interactions at different stages. We also
compare our results with a sample of radio-loud AGN and find a higher rate of
interaction signatures (95 +/- 21%) than in the type II quasars, but a very
similar range of surface brightnesses for the morphological features (20.9-24.8
mag arcsec sq), possibly indicating a similarity in the types of triggering
interactions.
  The range of features detected in the type II quasars suggests that AGN
activity can be triggered before, during or after the coalescence of the black
holes. Overall, our results are consistent with the idea that galaxy
interaction plays an important role in the triggering of quasar activity.
