A coincidence of disturbed morphology and blue UV colour: minor-merger
  driven star formation in early-type galaxies at z~0.6
  We exploit multi-wavelength photometry of early-type galaxies (ETGs) in the
COSMOS survey to demonstrate that the low-level star formation activity in the
ETG population at intermediate redshift is likely to be driven by minor
mergers. Splitting the ETGs into galaxies that show disturbed morphologies
indicative of recent merging and those that appear relaxed, we find that ~32%
of the ETG population appears to be morphologically disturbed. While the
relaxed objects are almost entirely contained within the UV red sequence, their
morphologically disturbed counterparts dominate the scatter to blue UV colours,
regardless of luminosity. Empirically and theoretically determined major-merger
rates in the redshift range z<1 are several times too low to account for the
fraction of disturbed ETGs in our sample, suggesting that minor mergers
represent the principal mechanism driving the observed star formation activity
in our sample. The young stellar components forming in these events have ages
between 0.03 and 0.3 Myrs and typically contribute <10% of the stellar mass of
the remnant. Together with recent work which demonstrates that the structural
evolution of nearby ETGs is consistent with one or more minor mergers, our
results indicate that the overall evolution of massive ETGs may be heavily
influenced by minor merging at late epochs and highlights the need to
systematically study this process in future observational surveys.
