The Metallicity Dilution in Local Massive Early-type Galaxies

We derive a sample of 114 Baldwin-Phillips-Terlevich-diagram star-forming and Wide-field Infrared Survey Explorer (WISE) low-star-formation-rate 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 (SDSS) Data Release 7 Max Planck Institute for Astrophysics-Johns Hopkins University emission-line measurements. We find that similar to 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.