AGN host galaxies at redshift z~0.7: peculiar or not?
  We perform a quantitative morphological comparison between the hosts of
Active Galactic Nuclei (AGN) and quiescent galaxies at intermediate redshifts
(z~0.7). The imaging data are taken from the large HST/ACS mosaics of the GEMS
and STAGES surveys. Our main aim is to test whether nuclear activity at this
cosmic epoch is triggered by major mergers. Using images of quiescent galaxies
and stars, we create synthetic AGN images to investigate the impact of an
optical nucleus on the morphological analysis of AGN hosts. Galaxy morphologies
are parameterized using the asymmetry index A, concentration index C, Gini
coefficient G and M20 index. A sample of ~200 synthetic AGN is matched to 21
real AGN in terms of redshift, host brightness and host-to-nucleus ratio to
ensure a reliable comparison between active and quiescent galaxies. The optical
nuclei strongly affect the morphological parameters of the underlying host
galaxy. Taking these effects into account, we find that the morphologies of the
AGN hosts are clearly distinct from galaxies undergoing violent gravitational
interactions. In fact, the host galaxies' distributions in morphological
descriptor space are more similar to undisturbed galaxies than major mergers.
Intermediate-luminosity (Lx < 10^44 erg/s) AGN hosts at z~0.7 show morphologies
similar to the general population of massive galaxies with significant bulges
at the same redshifts. If major mergers are the driver of nuclear activity at
this epoch, the signatures of gravitational interactions fade rapidly before
the optical AGN phase starts, making them undetectable on single-orbit HST
images, at least with usual morphological descriptors. This could be
investigated in future synthetic observations created from numerical
simulations of galaxy-galaxy interactions.
