Estimating the metallicity of star-forming early-type galaxies

Abstract

ABSTRACT We derive data for 4615 star-forming early-type galaxies (ETGs), which come from a cross-match of the Galaxy Zoo and the catalogue of the Max Planck Institute for Astrophysics–Johns Hopkins University (MPA–JHU) emission-line measurements for the Sloan Digital Sky Survey Data Release 7. Our sample is distributed mainly at $-0.7< \log({\rm SFR}/({\rm M}_{\odot }\, {\rm yr}^{-1})) < 1.2$, and the median value of our star formation rates (SFRs) is slightly higher than that shown in Davis & Young. We show that there is a significant trend of lower/higher stellar mass ETGs having lower/higher SFRs, and we obtain our sample best fit of $\log(\rm SFR/(M_{\odot} \rm yr^{-1}))=(0.74\pm0.01)\rm log(M_{*}/M_{\odot})-(7.64\pm0.10)$. We find the same slope as found by Cano-Díaz et al. In our star-forming ETG sample, we demonstrate clearly the correlation of the stellar mass–metallicity (MZ) relation. We find that higher metallicity measurements may be introduced by the diffuse ionized gas, when the D16, Sanch18 and Sander18 indicators are used to calibrate the metallicity of ETGs. We show the relations between the SFR and 12 + log(O/H) with different metallicity estimators, and we suggest that their correlations may be a consequence of the SFR–stellar mass and MZ relations in ETGs.