  The kinematic disturbances associated with major galaxy mergers are known to
produce gas inflows, which in turn may trigger accretion onto the supermassive
black holes (SMBH) of the participant galaxies. While this effect has been
studied in galaxy pairs, the frequency of active galactic nuclei (AGN) in fully
coalesced post-merger systems is poorly constrained due to the limited size or
impurity of extant post-merger samples. Previously, we combined convolutional
neural network (CNN) predictions with visual classifications to identify a
highly pure sample of 699 post-mergers in deep r-band imaging. In the work
presented here, we quantify the frequency of AGN in this sample using three
metrics: optical emission lines, mid-infrared (mid- IR) colour, and radio
detection of low-excitation radio galaxies (LERGs). We also compare the
frequency of AGN in post-mergers to that in a sample of spectroscopically
identified galaxy pairs. We find that AGN identified by narrow-line optical
emission and mid-IR colour have an increased incidence rate in post-mergers,
with excesses of ~4 over mass- and redshift-matched controls. The optical and
mid-IR AGN excesses in post-mergers exceed the values found for galaxy pairs,
indicating that AGN activity in mergers peaks after coalescence. Conversely, we
recover no significant excess of LERGs in post-mergers or pairs. Finally, we
find that the [OIII] luminosity (a proxy for SMBH accretion rate) in
post-mergers that host an optical AGN is ~0.3 dex higher on average than in
non-interacting galaxies with an optical AGN, suggesting that mergers generate
higher accretion rates than secular triggering mechanisms.
