  Here we use the Horizon-AGN simulation to test whether the spins of SMBHs in
merger-free galaxies are higher. We select samples using an observationally
motivated bulge-to-total mass ratio of < 0.1, along with two simulation
motivated thresholds selecting galaxies which have not undergone a galaxy
merger since z = 2, and those SMBHs with < 10% of their mass due to SMBH
mergers. We find higher spins (> 5{\sigma} ) in all three samples compared to
the rest of the population. In addition, we find that SMBHs with their growth
dominated by BH mergers following galaxy mergers, are less likely to be aligned
with their galaxy spin than those that have grown through accretion in the
absence of galaxy mergers (3.4{\sigma} ). We discuss the implications this has
for the impact of active galactic nuclei (AGN) feedback, finding that
merger-free SMBHs spend on average 91% of their lifetimes since z = 2 in a
radio mode of feedback (88% for merger-dominated galaxies). Given that previous
observational and theoretical works have concluded that merger-free processes
dominate SMBH-galaxy co-evolution, our results suggest that this co-evolution
could be regulated by radio mode AGN feedback.
