Strong inflows of gas from the outer disk to the inner kiloparsecsare induced during the interaction of disk galaxies. This inflow ofrelatively low-metallicity gas dilutes the metallicityof the circumnuclear gas. This process is critical for the galaxyevolution. We have investigated several aspects of the process as thetiming and duration of the dilution and its correlation with theinduced star formation. We analysed major(1:1) gas-richinteractions and mergers, spanning a range of initial orbitalcharacteristics. Star formation and metal enrichment from SNe areincludedin our model. Our results show that the strongest trend is between thestar formation rate and the dilution of the metals in the nuclearregion; i.e., the more intense the central burst of star formation, themore the gas is diluted. This trend comes from strong inflows ofrelatively metal-poor gas from the outer regions of both disks, whichfuels the intense star formation and lowers the overall metallicity fora time. The strong inflows happen on timescales of about 108yearsor less (i.e., on an internal dynamical time of the disk in thesimulations), and the most intense star formation and lowest gas phasemetallicities are seen generally after the first pericentre passage.Asthe star formation proceeds and the merger advances, thedilution reduces and enrichment becomes dominant- ultimatelyincreasing the metallicity of the circumnuclear gas to a level higherthan the initial metallicities of the merging galaxies. The``fly-bys''- pairs that interact but do not merge- alsocause some dilution. We even see some dilution early in the merger orin the ``fly-bys'' and thus do not observe a strong trend between thenuclear metallicities andseparation in our simulations until the merger is well advanced. Wealso analyse the O and Feenrichment of the ISM, and show that theevolution of the ratios, aswell as the dilution of the central gas metallicity, can be used as a clock for dating the interaction. Key words: galaxies: interactions - galaxies: formation - galaxies: evolution
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