The Connection Between the Growth of Massive Black Holes and Their Dynamical Histories
Massive black holes (MBHs) grow through both gas accretion and mergers and it remains unclear how variations to MBH growth and dynamical histories contribute to the spread in empirical scaling relations between MBHs and their host galaxies. We examine the ex- and in-situ growth of MBHs within large-scale cosmological simulations and compare the evolution of an outlier population of under-grown MBHs to the general population. We use the Romulus25 uniform volume cosmological simulation and the RomulusC cluster zoom-in simulation to examine the growth and merger histories of MBHs. We identify central MBHs that are undergrown relative to other MBHs in similar mass galaxies and investigate their accretion, merger, and orbital histories within the simulations. The contribution of ex-situ growth increases with MBH mass, with mergers becoming increasingly important in the most massive galaxies. We find no difference between cluster and field environments in terms of the importance of ex-situ growth, which is generally very low. Under-grown MBHs have experienced fewer mergers and have had complex dynamical histories, spending significant time 'drifting' within the galaxy or 'competing' with another MBH. Other under-grown MBHs have experienced a 'late entrance' into a galaxy which until then had no central MBH. Our results demonstrate that MBH assembly is dominated by in-situ growth, which in turn is impacted by the dynamical histories of MBHs within assembling galaxies. Prolonged off-centre evolution can suppress accretion and contribute to the formation of under-grown central MBHs at $z=0$.