Star formation and nuclear activity in close pairs of early-type
  galaxies
  We extract from the Sloan Digital Sky Survey a sample of 347 systems
involving early type galaxies separated by less than 30 kpc, in projection, and
500 km/s in radial velocity. These close pairs are likely progenitors of dry
mergers. The (optical) spectra is used to determine how the interaction affects
the star formation history and nuclear activity of the galaxies. The emission
lines (or lack thereof) are used to classify the sample into AGN, star forming
or quiescent. Increased AGN activity and reduced star formation in early-type
pairs that already appear to be interacting indicate that the merging process
changes the nature of nebular activity, a finding that is also supported by an
increase in AGN luminosity with decreasing pair separation. Recent star
formation is studied on the absorption line spectra, both through principal
component analysis as well as via a comparison of the spectra with composite
stellar population models. We find that the level of recent star formation in
close pairs is raised relative to a control sample of early-type galaxies. This
excess of residual star formation is found throughout the sample of close pairs
and does not correlate with pair separation or with visual signs of
interaction. Our findings are consistent with a scenario whereby the first
stage of the encounter (involving the outer parts of the halos) trigger
residual star formation, followed by a more efficient inflow towards the centre
-- switching to an AGN phase -- after which the systems are quiescent.
