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% of the budget, while Sb/Sc galaxies host the bulk
(~53%) 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% 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 because a bigger bulge better
stabilizes the disk 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%. Thus, a
lower limit for the fraction of cosmic star formation that is induced by minor
mergers is ~35% (14% [ETGs] + 0.24 x 86% [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.
