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Understanding the processes that drive the formation of black holes (BHs) isa key topic in observational cosmology. While the observed$M_{\mathrm{BH}}$--$M_{\mathrm{Bulge}}$ correlation in bulge-dominated galaxiesis thought to be produced by major mergers, the existence of a$M_{\mathrm{BH}}$--$M_{\star}$ relation, across all galaxy morphological types,suggests that BHs may be largely built by secular processes. Recent evidencethat bulge-less galaxies, which are unlikely to have had significant mergers,are offset from the $M_{\mathrm{BH}}$--$M_{\mathrm{Bulge}}$ relation, but lieon the $M_{\mathrm{BH}}$--$M_{\star}$ relation, has strengthened thishypothesis. Nevertheless, the small size and heterogeneity of current datasets,coupled with the difficulty in measuring precise BH masses, makes itchallenging to address this issue using empirical studies alone. Here, we useHorizon-AGN, a cosmological hydrodynamical simulation to probe the role ofmergers in BH growth over cosmic time. We show that (1) as suggested byobservations, simulated bulge-less galaxies lie offset from the main$M_{\mathrm{BH}}$--$M_{\mathrm{Bulge}}$ relation, but on the$M_{\mathrm{BH}}$--$M_{\star}$ relation, (2) the positions of galaxies on the$M_{\mathrm{BH}}$--$M_{\star}$ relation are not affected by their mergerhistories and (3) only $\sim$35 per cent of the BH mass in today's massivegalaxies is directly attributable to merging -- the majority ($\sim$65 percent) of BH growth, therefore, takes place gradually, via secular processes,over cosmic time.

Normal black holes in bulge-less galaxies: the largely quiescent, merger-free growth of black holes over cosmic time