Measuring the Metallicity of Early-type Galaxies. I. Composite Region

TL;DR

This study analyzes a large sample of early-type galaxies to measure their metallicities, compare different calibration methods, and explore the relationship between star formation rate and metallicity, revealing a weak positive correlation likely influenced by minor mergers.

Contribution

It introduces a comprehensive analysis of metallicity calibration methods for early-type galaxies and investigates the SFR-metallicity relationship in a large, composite galaxy sample.

Findings

Courti17 is the most accurate metallicity calibrator among four tested.

Most ETGs with star formation lie on the galaxy main sequence.

A weak positive correlation exists between SFR and metallicity when calibrated with certain indicators.

Abstract

We present the data of 9,739 early-type galaxies (ETGs), cross-matching the Galaxy Zoo 1 with our sample selected from the catalog of the Sloan Digital Sky Survey Data Release 7 of MPA-JHU emission-line measurements. We first investigate the divisor between ETGs with and without star formation (SF), and find the best separator of W2-W3=2.0. is added. We explore the ETG sample by refusing a varity of ionization sources, and derive 5376 ETGs with SF by utilizing a diagnostic tool of the division line of $W2-W3=2.0$. We measure their metallicities with four abundance calibrators. We find that our composite ETG sample has similar distributons of $M_{*}$ and star formation rate (SFR) as star-forming galaxies (SFGs) do, that most of them lie on the "main sequence", and that our fit is a slightly steeper slope than that derived in Renzini \& Peng. Compared with the distributions between different metallicities calibrated by four abundance indicators, we find that the Courti17 method is the most accurate calibrator for composite ETGs among the four abundance indicators. We present a weak positive correlation of SFR and metallicity only when the metallicity is calibrated by the PP04, Curti17, and T04 indicators. The correlation is not consistent with the negative correlation of both parameters in SFGs. We suggest that the weak correlation is due to the dilution effect of gas inflow driven by minor mergers.