Observational evidence for the evolution of nuclear metallicity and star formation rate as the merger stage

TL;DR

This study provides observational evidence that nuclear metallicity decreases during early merger stages and then increases at coalescence, with star formation rate enhancements correlating with metallicity evolution, supporting simulation predictions.

Contribution

It is the first observational study to track nuclear metallicity and star formation rate evolution across merger stages in local galaxies.

Findings

Metallicity decreases by ~0.06 dex in close interactions.

Star formation rate is enhanced during mergers.

ISM enrichment occurs at final coalescence.

Abstract

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.