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 Sloan Digital Sky Survey Data Release 7. The galaxy pairs are selected to have velocity differences Δv < 300 kms^-1, projected separations (r_p) < 80 h70-1kpc, 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 per cent higher than the control sample at r_p < 30 h70-1kpc. It is at these small separations that the strongest enhancements in SFR (by up to a factor of ∼10) are measured, although such starbursts are rare, even amongst the closest pairs. In addition, the pairs demonstrate more modest SFR enhancements of ∼30 per cent out to at least 80 h70-1kpc (the widest separations in our sample). This is the first time that enhanced SFRs have been robustly detected out to such large projected separations. Galaxies in both major and minor mergers show significant SFR enhancements at all r_p, although the strongest starbursts (with SFR enhancements of a factor of ∼10) appear to be found only in the major mergers. We also find evidence that SFR enhancements are synchronized in an interacting pair, such that a higher SFR in one galaxy is accompanied by an increased SFR in its companion. 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, a trend that mirrors the SFR enhancements as a function of r_p. 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 h70-1kpc 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.