The dilution peak, metallicity evolution, and dating of galaxy
  interactions and mergers
  Strong inflows of gas from the outer disk to the inner kiloparsecs are
induced during the interaction of disk galaxies. This inflow of relatively
low-metallicity gas dilutes the metallicity of the circumnuclear gas. We have
investigated several aspects of the process as the timing and duration of the
dilution and its correlation with the induced star formation. We analysed major
(1:1) gas-rich interactions and mergers, spanning a range of initial orbital
characteristics. Star formation and metal enrichment from SNe are included in
our model. Our results show that the strongest trend is between the star
formation rate and the dilution of the metals in the nuclear region; i.e., the
more intense the central burst of star formation, the more the gas is diluted.
This trend comes from strong inflows of relatively metal-poor gas from the
outer regions of both disks, which fuels the intense star formation and lowers
the overall metallicity for a time. The strong inflows happen on timescales of
about 10^8 years or less, and the most intense star formation and lowest gas
phase metallicities are seen generally after the first pericentre passage. As
the star formation proceeds and the merger advances, the dilution reduces and
enrichment becomes dominant - ultimately increasing the metallicity of the
circumnuclear gas to a level higher than the initial metallicities of the
merging galaxies. The "fly-bys" - pairs that interact but do not merge - also
cause some dilution. We even see some dilution early in the merger or in the
"fly-bys" and thus do not observe a strong trend between the nuclear
metallicities and separation in our simulations until the merger is well
advanced. We also analyse the O and Fe enrichment of the ISM, and show that the
evolution of the alpha/Fe ratios, as well as the dilution of the central gas
metallicity, can be used as a clock for "dating" the interaction.
