The importance of minor-merger-driven star formation and black-hole
  growth in disk galaxies
  We use the SDSS Stripe 82 to empirically quantify the stellar-mass and
black-hole growth triggered by minor mergers in local spiral (disk) galaxies.
Since major mergers destroy disks 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 ~40% 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 vs.
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 ~14% 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.
