Galaxy pairs in the Sloan Digital Sky Survey - V. Tracing changes in star formation rate and metallicity out to separations of 80 kpc

Abstract

We present a sample of 1899 galaxies with a close companion taken from the\nSDSS DR7. The galaxy pairs are selected to have velocity differences < 300\nkm/s, projected separations (rp) < 80 kpc/h, mass ratios between 0.1 and 10,\nand robust measurements of star formation rates and gas-phase metallicities. We\nmatch the galaxies in total stellar mass, redshift, and local density to a set\nof 10 control galaxies per pair galaxy. For each pair galaxy we can therefore\ncalculate the statistical change in star formation rate (SFR) and metallicity\nassociated with the interaction process. Relative to the control sample, we\nfind that galaxies in pairs show typical SFR enhancements that are, on average,\n60% higher than the control sample at rp < 30 kpc/h. In addition, the pairs\ndemonstrate more modest SFR enhancements of ~30% out to at least 80 kpc/h (the\nwidest separations in our sample). Galaxies in both major and minor mergers\nshow significant SFR enhancements at all rp, although the strongest starbursts\n(with SFR enhancements of a factor of ~10) appear to be found only in the major\nmergers. For the first time, we are also able to trace the metallicity changes\nin galaxy pairs as a function of projected separation. The metallicity is\ngenerally diluted in galaxy pairs by ~0.02 dex, with an average metallicity\ndecrement of -0.03 dex at the smallest separations. The SFR and metallicity\ntrends with projected separation are interpreted through a comparison with\ntheoretical models. These simulations indicate that the peak in SFR\nenhancements at small separations is due to systems near the end of the merger\nprocess. The extended plateau in SFR enhancements out to at least 80 kpc/h is\ndominated by galaxies that have made a pericentric passage and are now\nexperiencing triggered star formation on their trajectory towards\napogalacticon, or on a subsequent close approach. (Abridged)\n