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What is the relevance of major mergers and interactions as triggeringmechanisms for active galactic nuclei (AGN) activity? To answer thislongstanding question, we analyze 140 XMM-selected AGN host galaxies and amatched control sample of 1264 inactive galaxies over z~0.3-1.0 andlog(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 classifiersyields a measure of the fraction of distorted morphologies in the AGN andcontrol samples, i.e. quantifying the signature of recent mergers which mightpotentially be responsible for fueling/triggering the AGN. We find that (1) thevast majority (>85%) of the AGN host galaxies do not show strong distortions,and (2) there is no significant difference in the distortion fractions betweenactive and inactive galaxies. Our findings provide the best direct evidencethat, since z~1, the bulk of black hole accretion has not been triggered bymajor galaxy mergers, therefore arguing that the alternative mechanisms, i.e.,secular processes and minor interactions, are the leading triggers for theepisodes of major black hole growth. We also exclude an alternativeinterpretation of our results: a significant time lag between merging and theobservability of the AGN phase could wash out the most significant mergingsignatures, explaining the lack of enhancement of strong distortions on the AGNhosts. We show that this alternative scenario is unlikely due to: (1) recentmajor mergers being ruled out for the majority of sources due to the highfraction of disk-hosted AGN, (2) the lack of a significant X-ray signal inmerging inactive galaxies as a signature of a potential buried AGN, and (3) thelow levels of soft X-ray obscuration for AGN hosted by interacting galaxies, incontrast to model predictions.

The bulk of the black hole growth since z~1 occurs in a secular universe: No major merger-AGN connection