The metallicity dilution in local massive early-type galaxies

TL;DR

This study identifies that about 28% of local massive early-type galaxies show significant metallicity dilution, likely caused by inflow of metal-poor gas from mergers or the intergalactic medium, affecting their star formation and color.

Contribution

It provides the first large-scale analysis linking metallicity dilution in early-type galaxies to gas inflow from mergers or the intergalactic medium.

Findings

28% of ETGs have metallicity at least 2σ below the MZ relation.

Diluted ETGs tend to have lower SFR and redder colors.

Metallicity dilution is likely due to inflow of metal-poor gas.

Abstract

We derive a sample of 114 Baldwin-Phillips-Terlevich diagram - star formation (BPT-SF) and Wide-field infrared Survey Exploer - low star formation rate (WISE-LSFR) early-type galaxies (ETGs) by utilizing the criterion W2-W3$<2.5$ (where W2 and W3 are the wavelengths of 4.6 and 12 $\mu m$ in the WISE four bands) and cross-matching the $Galaxy~Zoo~1$ and the catalog of the Sloan Digital Sky Survey Data SDSS Release 7 MPA-JHU emission-line measurements. We find that \textbf{$\sim 28\%$} of our ETGs exhibit a metallicity that is at least 2 standard deviation (0.26 dex) below the mass-metallicity (MZ) relation of star-forming galaxies (SFGs) from the SDSS. We demonstrate that almost all of our ETGs locate below the ``main sequence'' of SFGs. We find that these ETGs with larger metallicity deviation from the MZ relation tend to have lower SFR and redder color. By exploring the dilution properties of these massive ETGs, we report that the dilution effect may be mainly attributed to the inflow of metal-poor gas from mergers/interaction or the intergalactic medium.