Galaxy Pairs in the Sloan Digital Sky Survey - V. Tracing changes in
  star formation rate and metallicity out to separations of 80 kpc
  We present a sample of 1899 galaxies with a close companion taken from the
SDSS DR7. The galaxy pairs are selected to have velocity differences < 300
km/s, projected separations (rp) < 80 kpc/h, mass ratios between 0.1 and 10,
and robust measurements of star formation rates and gas-phase metallicities. We
match the galaxies in total stellar mass, redshift, and local density to a set
of 10 control galaxies per pair galaxy. For each pair galaxy we can therefore
calculate the statistical change in star formation rate (SFR) and metallicity
associated with the interaction process. Relative to the control sample, we
find that galaxies in pairs show typical SFR enhancements that are, on average,
60% higher than the control sample at rp < 30 kpc/h. In addition, the pairs
demonstrate more modest SFR enhancements of ~30% out to at least 80 kpc/h (the
widest separations in our sample). Galaxies in both major and minor mergers
show significant SFR enhancements at all rp, although the strongest starbursts
(with SFR enhancements of a factor of ~10) appear to be found only in the major
mergers. For the first time, we are also able to trace the metallicity changes
in galaxy pairs as a function of projected separation. The metallicity is
generally diluted in galaxy pairs by ~0.02 dex, with an average metallicity
decrement of -0.03 dex at the smallest separations. The SFR and metallicity
trends with projected separation are interpreted through a comparison with
theoretical models. These simulations indicate that the peak in SFR
enhancements at small separations is due to systems near the end of the merger
process. The extended plateau in SFR enhancements out to at least 80 kpc/h is
dominated by galaxies that have made a pericentric passage and are now
experiencing triggered star formation on their trajectory towards
apogalacticon, or on a subsequent close approach. (Abridged)
