Metallicity and Nuclear Star Formation in Nearby Galaxy Pairs: Evidence for Tidally Induced Gas Flows

We derive the first luminosity-metallicity relation for a large objectivelyselected sample of local galaxy pairs and we compare the pairs LZ relation withthe relation for the Nearby Field Galaxy Survey (NFGS). Galaxy pair memberswith small projected separations (s<20 kpc/h) have systematically lowermetallicities (~0.2 dex on average) than the field galaxies, or than morewidely separated pairs at the same luminosity. There is a strong correlationbetween metallicity and central burst strength in the galaxy pairs. All fivegalaxies in the pairs sample with strong central bursts have close companionsand metallicities lower than the comparable field galaxies. Our results providestrong observational evidence for a merger scenario where galaxy interactionscause gas flows towards the central regions, carrying less enriched gas fromthe outskirts of the galaxy into the central regions. The less enriched gasdilutes the pre-existing nuclear gas to produce a lower metallicity than wouldbe obtained prior to the interaction. These gas flows trigger central bursts ofstar formation, causing strong central burst strengths, and possibly aiding theformation of blue bulges. We show that the timescale and central gas dilutionrequired by this scenario are consistent with predictions from hydrodynamicmerger models.Comment: 16 pages, 15 figures. Submitted to A