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Recent observational and theoretical studies have suggested that supermassive black holes (SMBHs) grow mostly through non-merger (‘secular’) processes. Since galaxy mergers lead to dynamical bulge growth, the only way to observationally isolate non-merger growth is to study galaxies with low bulge-to-total mass ratio (e.g. $B/T\lt 10~{{\ \rm per\ cent}}$). However, bulge growth can also occur due to secular processes, such as disc instabilities, making disc-dominated selections a somewhat incomplete way to select merger-free systems. Here we use the Horizon-AGN simulation to select simulated galaxies which have not undergone a merger since z = 2, regardless of bulge mass, and investigate their location on typical black hole-galaxy scaling relations in comparison to galaxies with merger dominated histories. While the existence of these correlations has long been interpreted as co-evolution of galaxies and their SMBHs driven by galaxy mergers, we show here that they persist even in the absence of mergers. We find that the correlations between SMBH mass and both total mass and stellar velocity dispersion are independent of B/T ratio for both merger-free and merger-dominated galaxies. In addition, the bulge mass and SMBH mass correlation is still apparent for merger-free galaxies, the intercept for which is dependent on B/T. Galaxy mergers reduce the scatter around the scaling relations, with merger-free systems showing broader scatter. We show that for merger-free galaxies, the co-evolution is dominated by radio-mode feedback, and suggest that the long periods of time between galaxy mergers make an important contribution to the co-evolution between galaxies and SMBHs in all galaxies.

Here, we use the Horizon–active galactic nucleus (AGN) simulation to test whether the spins of supermassive black hole (SMBH) 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 $\lt 10~{{\ \rm per\ cent}}$ of their mass due to SMBH mergers. We find higher spins (>5σ) in all three sample 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 (3.4σ). We discuss the implications this has for the impact of active galactic nucleus (AGN) feedback, finding that merger-free SMBHs spend on average 91 per cent of their lifetimes since z = 2 in a radio mode of feedback (88 per cent 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.