Metallicity and nuclear star formation in nearby galaxy pairs: Evidence for tidally induced gas flows

We derive the first luminosity-metallicity (L-Z) relation for a large objectively selected sample of local galaxy pairs, and we compare the L-Z relation of the pairs with the relation for the Nearby Field Galaxy Survey. Galaxy pair members with small projected separations (s < 20 kpc h(-1)) have systematically lower metallicities (similar to 0.2 dex on average) than either the field galaxies or more widely separated pairs at the same luminosity. There is a strong correlation between metallicity and central burst strength in the galaxy pairs. All five galaxies in the pairs sample with strong central bursts have close companions and metallicities lower than the comparable field galaxies. Our results provide strong observational evidence for a merger scenario in which galaxy interactions cause gas flows toward the central regions, carrying less enriched gas from the outskirts of the galaxy into the central regions. The less enriched gas dilutes the preexisting nuclear gas to produce a lower metallicity than would be obtained prior to the interaction. These gas flows trigger central bursts of star formation, causing strong central burst strengths and possibly aiding the formation of blue bulges. We show that the timescale and central gas dilution required by this scenario are consistent with predictions from hydrodynamic merger models.