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

TL;DR

This study analyzes 1899 galaxy pairs from SDSS DR7 to understand how star formation rates and metallicity change during galaxy interactions up to 80 kpc separation, revealing significant SFR enhancements and metallicity dilution.

Contribution

It provides the first detailed measurement of metallicity changes as a function of galaxy separation during interactions, and links observed trends to theoretical merger models.

Findings

Galaxies in pairs show 60% higher SFR at rp < 30 kpc/h.

SFR enhancements extend out to at least 80 kpc/h, averaging 30%.

Metallicity is diluted by ~0.02 dex in galaxy pairs.

Abstract

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)