Galaxy merger histories and the role of merging in driving star formation at z > 1

We use Horizon-AGN, a hydrodynamical cosmological simulation, to explore the role of mergers in the evolution of massive (M-* > 10(10) M-circle dot) galaxies around the epoch of peak cosmic star formation (1 < z < 4). The fraction of massive galaxies in major mergers (mass ratio R < 4: 1) is around 3 per cent, a factor of similar to 2.5 lower than minor mergers (4: 1 < R < 10: 1) at these epochs, with no trend with redshift. At z similar to 1, around a third of massive galaxies have undergone a major merger, while all remaining systems have undergone a minor merger. While almost all major mergers at z > 3 are 'blue' (i.e. have significant associated star formation), the proportion of 'red' mergers increases rapidly at z < 2, with most merging systems at z similar to 1.5 producing remnants that are red in rest-frame UV-optical colours. The star formation enhancement during major mergers is mild (similar to 20-40 per cent) which, together with the low incidence of such events, implies that this process is not a significant driver of early stellar mass growth. Mergers (R < 10: 1) host around a quarter of the total star formation budget in this redshift range, with major mergers hosting around two-thirds of this contribution. Notwithstanding their central importance to the standard Lambda cold dark matter paradigm, mergers are minority players in driving star formation at the epochs where the bulk of today's stellar mass was formed.