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 \Delta v < 300 km s ^ { -1 } , projected separations ( r _ { p } ) < 80 h _ { 70 } ^ { -1 } kpc , 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 r _ { p } < 30 h _ { 70 } ^ { -1 } kpc .
It is at these small separations that the strongest enhancements in SFR ( by up to a factor \sim 10 ) are measured , although such starbursts are rare , even amongst the closest pairs .
In addition , the pairs demonstrate more modest SFR enhancements of \sim 30 % out to at least 80 h _ { 70 } ^ { -1 } kpc ( 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 \sim 10 ) appear to be found only in the major mergers .
We also find evidence that SFR enhancements are synchronised 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 \sim 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 h _ { 70 } ^ { -1 } kpc 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 .