The significant contribution of minor mergers to the cosmic star formation budget

We estimate an empirical lower limit for the fraction of cosmic star formation that is triggered by minor mergers in the local Universe. Splitting the star formation budget by galaxy morphology, we find that early-type galaxies (ETGs) host ~14 per cent of the budget, while Sb/Sc galaxies host the bulk (~53 per cent) of the local star formation activity. Recent work indicates that star formation in nearby ETGs is driven by minor mergers, implying that at least ~14 per cent of local star formation is triggered by this process. A more accurate estimate can be derived by noting that an infalling satellite likely induces a larger starburst in a galaxy of 'later' morphological type, both due to higher availability of gas in the accreting galaxy and also because a bigger bulge better stabilizes the disc against star formation. This enables us to use the star formation in ETGs to estimate a lower limit for the fraction of star formation in late-type galaxies (LTGs) that is minor-merger-driven. Using a subsample of ETGs that is mass-and environment-matched to the LTGs (implying a similar infalling satellite population), we estimate this limit to be ~24 per cent. Thus, a lower limit for the fraction of cosmic star formation that is induced by minor mergers is ~35 per cent [14 per cent (ETGs) + 0.24 × 86 per cent (LTGs)]. The observed positive correlation between black hole and galaxy mass further implies that a similar fraction of black hole accretion may also be triggered by minor mergers. Detailed studies of minor-merger remnants are therefore essential, to quantify the role of this important process in driving stellar mass and black hole growth in the local Universe.