Normal black holes in bulge-less galaxies: the largely quiescent,
  merger-free growth of black holes over cosmic time
  Understanding the processes that drive the formation of black holes (BHs) is
a key topic in observational cosmology. While the observed
$M_{\mathrm{BH}}$--$M_{\mathrm{Bulge}}$ correlation in bulge-dominated galaxies
is 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 evidence
that bulge-less galaxies, which are unlikely to have had significant mergers,
are offset from the $M_{\mathrm{BH}}$--$M_{\mathrm{Bulge}}$ relation, but lie
on the $M_{\mathrm{BH}}$--$M_{\star}$ relation, has strengthened this
hypothesis. Nevertheless, the small size and heterogeneity of current datasets,
coupled with the difficulty in measuring precise BH masses, makes it
challenging to address this issue using empirical studies alone. Here, we use
Horizon-AGN, a cosmological hydrodynamical simulation to probe the role of
mergers in BH growth over cosmic time. We show that (1) as suggested by
observations, 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 merger
histories and (3) only $\sim$35 per cent of the BH mass in today's massive
galaxies is directly attributable to merging -- the majority ($\sim$65 per
cent) of BH growth, therefore, takes place gradually, via secular processes,
over cosmic time.
