  What is the relevance of major mergers and interactions as triggering
mechanisms for active galactic nuclei (AGN) activity? To answer this
longstanding question, we analyze 140 XMM-selected AGN host galaxies and a
matched control sample of 1264 inactive galaxies over z~0.3-1.0 and
log(M_*/M_sun)<11.7 with high-resolution HST/ACS imaging from the COSMOS field.
The visual analysis of their morphologies by 10 independent human classifiers
yields a measure of the fraction of distorted morphologies in the AGN and
control samples, i.e. quantifying the signature of recent mergers which might
potentially be responsible for fueling/triggering the AGN. We find that (1) the
vast majority (>85%) of the AGN host galaxies do not show strong distortions,
and (2) there is no significant difference in the distortion fractions between
active and inactive galaxies. Our findings provide the best direct evidence
that, since z~1, the bulk of black hole accretion has not been triggered by
major galaxy mergers, therefore arguing that the alternative mechanisms, i.e.,
secular processes and minor interactions, are the leading triggers for the
episodes of major black hole growth. We also exclude an alternative
interpretation of our results: a significant time lag between merging and the
observability of the AGN phase could wash out the most significant merging
signatures, explaining the lack of enhancement of strong distortions on the AGN
hosts. We show that this alternative scenario is unlikely due to: (1) recent
major mergers being ruled out for the majority of sources due to the high
fraction of disk-hosted AGN, (2) the lack of a significant X-ray signal in
merging inactive galaxies as a signature of a potential buried AGN, and (3) the
low levels of soft X-ray obscuration for AGN hosted by interacting galaxies, in
contrast to model predictions.
