Observational evidence for the evolution of nuclear metallicity and star
  formation rate as the merger stage
  We investigate the evolution of nuclear gas-phase oxygen abundance and star
formation rate (SFR) of local far-infrared selected star-forming galaxies along
the merger sequence, as traced by their optical morphologies. The sample was
drawn from a cross-correlation analysis of the IRAS Point Source Catalog
Redshift Survey and 1 Jy ultraluminous infrared galaxies sample with the Sloan
Digital Sky Survey Data Release 7 database. The investigation is done by
comparing our sample to a control sample matched in the normalized redshift
distribution in two diagnostics, which are the nuclear gas-phase metallicity
vs. stellar mass and the nuclear SFR vs. stellar mass diagrams. Galaxies with
different morphological types show different mass-metallicity relations (MZR).
Compared to the MZR defined by the control sample, isolated spirals have
comparable metallicities with the control sample at a given stellar mass.
Spirals in pairs and interacting galaxies with projected separations $r_{p} >$
20 kpc show mild metallicity dilution of 0.02-0.03 dex. Interacting galaxies
with $r_{p} <$ 20 kpc, pre-mergers and advanced mergers are under-abundant by
~0.06, ~0.05 and ~0.04 dex, respectively. This shows an evolutionary trend that
the metallicity is increasingly depressed as the merging proceeds and it is
diluted most dramatically when two galaxies are closely interacting.
Afterwards, the interstellar medium (ISM) is enriched when the galaxies
coalesce. This is the first time that such ISM enrichment at the final
coalescence stage is observed, which demonstrates the importance of supernova
explosion in affecting the nuclear metallicity. Meanwhile the central SFR
enhancement relative to the control sample evolves simultaneously with the
nuclear gas-phase oxygen abundance. Our results support the predictions from
numerical simulations.
