The importance of minor-merger-driven star formation and black hole growth in disc galaxies

We use the Sloan Digital Sky Survey Stripe 82 to empirically quantify the stellar-mass and black hole growth triggered by minor mergers in local spiral (disc) galaxies. Since major mergers destroy discs and create spheroids, morphologically disturbed spirals are likely remnants of minor mergers. Disturbed spirals exhibit enhanced specific star formation rates (SSFRs), the enhancement increasing in galaxies of 'later' morphological type (which have more gas and smaller bulges). By combining the SSFR enhancements with the fraction of time spirals spend in this 'enhanced' mode, we estimate that similar to 40 per cent of the star formation in local spirals is directly triggered by minor mergers. The disturbed spirals also exhibit higher nuclear-accretion rates, implying that minor mergers enhance the growth rate of the central black hole. However, the specific accretion rate shows a lower enhancement than that in the SSFR, suggesting that the coupling between stellar-mass and black hole growth is weak in minor-merger-driven episodes. Given the significant fraction of star formation that is triggered by minor mergers, this weaker coupling may contribute to the large intrinsic scatter observed in the stellar versus black hole mass relation in spirals. Combining our results with the star formation in early-type galaxies - which is minor-merger-driven and accounts for similar to 14 per cent of the star formation budget - suggests that around half of the star formation activity in the local Universe is triggered by the minor-merger process.