The limited role of galaxy mergers in driving stellar mass growth over
  cosmic time
  A key unresolved question is the role that galaxy mergers play in driving
stellar mass growth over cosmic time. Recent observational work hints at the
possibility that the overall contribution of `major' mergers (mass ratios
$\gtrsim$1:4) to cosmic stellar mass growth may be small, because they enhance
star formation rates by relatively small amounts at high redshift, when much of
today's stellar mass was assembled. However, the heterogeneity and relatively
small size of today's datasets, coupled with the difficulty in identifying
genuine mergers, makes it challenging to $\textit{empirically}$ quantify the
merger contribution to stellar mass growth. Here, we use Horizon-AGN, a
cosmological hydrodynamical simulation, to comprehensively quantify the
contribution of mergers to the star formation budget over the lifetime of the
Universe. We show that: (1) both major and minor mergers enhance star formation
to similar amounts, (2) the fraction of star formation directly attributable to
merging is small at all redshifts (e.g. $\sim$35 and $\sim$20 per cent at
z$\sim$3 and z$\sim$1 respectively) and (3) only $\sim$25 per cent of today's
stellar mass is directly attributable to galaxy mergers over cosmic time. Our
results suggest that smooth accretion, not merging, is the dominant driver of
stellar mass growth over the lifetime of the Universe.
