The insignificance of major mergers in driving star formation at z~2
  We study the significance of major-merger-driven star formation in the early
Universe, by quantifying the contribution of this process to the total star
formation budget in 80 massive (M* > 10^10 MSun) galaxies at z~2. Employing
visually-classified morphologies from rest-frame V-band HST imaging, we find
that 55+/-14% of the star formation budget is hosted by non-interacting
late-types, with 27+/-8% in major mergers and 18+/-6% in spheroids. Given that
a system undergoing a major merger continues to experience star formation
driven by other processes at this epoch (e.g. cold accretion, minor mergers),
~27% is an upper limit to the major-merger contribution to star formation
activity at this epoch. The ratio of the average specific star formation rate
in major mergers to that in the non-interacting late-types is ~2.2:1,
suggesting that the enhancement of star formation due to major merging is
typically modest, and that just under half the star formation in systems
experiencing major mergers is unrelated to the merger itself. Taking this into
account, we estimate that the actual major-merger contribution to the star
formation budget may be as low as ~15%. While our study does not preclude a
major-merger-dominated era in the very early Universe, if the major-merger
contribution to star formation does not evolve strongly into larger look-back
times, then this process has a relatively insignificant role in driving stellar
mass assembly over cosmic time.
