The insignificance of major mergers in driving star formation at z ≃ 2

Abstract

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* > 1010 M) galaxies at z ≃ 2. Employing visually classified morphologies from restframe V-band Hubble Space Telescope (HST) imaging, we find that 55 per cent of the star formation budget is hosted by non-interacting late types, with 27 per cent in major mergers and 18±6 per cent in spheroids. Given that a system undergoing a major merger continues toexperience star formation driven by other processes at this epoch (e.g. cold accretion and minor mergers), ~27 per cent 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 tomajor merging is typically modest, and that just under half the star formation in systemsexperiencing 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 per cent. 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 processhas a relatively insignificant role in driving stellar mass assembly over cosmic time.