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