The redshift evolution of major merger triggering of luminous AGN: a
  slight enhancement at z$\sim$2
  Active galactic nuclei (AGN), particularly the most luminous AGN, are
commonly assumed to be triggered through major mergers, however observational
evidence for this scenario is mixed. To investigate any influence of galaxy
mergers on AGN triggering and luminosities through cosmic time, we present a
sample of 106 luminous X-ray selected type 1 AGN from the COSMOS survey. These
AGN occupy a large redshift range (0.5 < z < 2.2) and two orders of magnitude
in X-ray luminosity ($\sim$10$^{43}$ - 10$^{45}$ erg s$^{-1}$). AGN hosts are
carefully mass and redshift matched to 486 control galaxies. A novel technique
for identifying and quantifying merger features in galaxies is developed,
subtracting GALFIT galaxy models and quantifying the residuals. Comparison to
visual classification confirms this measure reliably picks out disturbance
features in galaxies. No enhancement of merger features with increasing AGN
luminosity is found with this metric, or by visual inspection. We analyse the
redshift evolution of AGN associated with galaxy mergers and find no merger
enhancement in lower redshift bins. Contrarily, in the highest redshift bin
(z$\sim$2) AGN are $\sim$4 times more likely to be in galaxies exhibiting
evidence of morphological disturbance compared to control galaxies, at 99%
confidence level ($\sim$2.4$\sigma$) from visual inspection. Since only
$\sim$15% of these AGN are found to be in morphologically disturbed galaxies,
it is implied that major mergers at high redshift make a noticeable but
subdominant contribution to AGN fuelling. At low redshifts other processes
dominate and mergers become a less significant triggering mechanism.
