Abstract
Abstract We present the data of 9739 early-type galaxies (ETGs), cross-matching 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 variety 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 distributions of M * and star formation rate 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 Curti17 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.
Highlights
Early-type galaxies (ETGs) are generally thought to be “red and dead” objects, including ellliptical and lenticular galaxies, and contain little or no ongoing star formation (SF)
The value is 0.5 mag smaller than that proposed by Herpich et al (2016), and our value can be explained because the value of W2-W3=2.5 is suitable for all galaxy methologies in Herpich et al (2016), while our value is applied to all ETGs, for example, wAGN ETGs, lineless retired” (LLR) ETGs, and emission-line retired” (ELR) ETGs, generally having a smaller W2W3 color
We find that the distribution of our ETG sample is similar to the star-forming galaxies (SFGs) one, and most of our composite ETG sample lies on the “main sequence” (Noeske et al 2007; Salim et al 2007; Renzini & Peng 2015), but our sample is lower than the main sequence by 0.2-0.3 dex
Summary
Early-type galaxies (ETGs) are generally thought to be “red and dead” objects, including ellliptical and lenticular galaxies, and contain little or no ongoing star formation (SF). Belli et al (2017) found that 9 of 20 quiescent galaxies present evidence of SF activity, and suggested that the low-level activity may be fueled by infalling gas or minor mergers. The gas-phase oxygen abundance (metallicity) is a key fundamental parameter in galaxy formation and evolution, and the metallicity is critical for studying the properties of ETGs. Some studies try to calibrate the metallicity for the H II region of ETGs. Using a sample of quiescent red sequence from the SDSS, Yan (2018) attempted to calibrate the metallicity in galaxies dominated by diffuse ionized gas (DIG)/low-ionization emission region emission, and presented some puzzles, showing that his results are lower than those calibrated by other metallicity indicators.
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