| Abstract
| - Aims. We study the stellar population of local infrared galaxies, which contain star-forming galaxies, composite galaxies, LINERs, and Seyfert 2s. We also want to find whether infrared luminosity and spectral class have any effect on their stellar populations. Methods. The sample galaxies are selected from the main galaxy sample of SDSS-DR4 and then cross-correlated with the IRAS-PSCz catalog. We fit our spectra (stellar absorption lines and continua) using the spectral synthesis code STARLIGHT based on the templates of simple stellar population and the spectra of star clusters. Results. Among the 4 spectral classes, LINERs present the oldest stellar populations, and the other 3 subsamples all present substantial young and intermediate age populations and very few old populations. The importance of young populations decreases from star-forming, composite, Seyfert 2 to LINER. As for the different infrared luminosity bins, ULIGs & LIGs (log( $L_{\rm IR}/L_{odot})$ ≥ 11) present younger populations than do starbursts and normal galaxies; however, the dominant contributors to mass are old populations in all sample galaxies. The fittings also give consistent results by using the spectra of star clusters with different ages and metallicities as templates. The dominated populations in star-forming and composite galaxies are those with metallicity $Z=0.2~Z_odot$, while LINERs and Seyfert 2s are more metal-rich. The normal galaxies are more metal-rich than the ULIGs & LIGs and starbursts for the star-forming galaxies within different infrared luminosity bins.
Additionally, we compare some synthesis results with other parameters obtained from the MPA/JHU catalog. First we find that the stellar and nebular extinctions are correlated, and the ionized gas suffers twice as much extinction as stars. Second we confirm that $D_{n}(4000)$ is a much better age indicator than $H\delta_{A}$. Following the evolution of galaxies, $D_{n}(4000)$ monotonously varies. Finally we investigate some relationships between mean stellar age, mean stellar metallicity, and nebular metallicity for the subsample of star-forming galaxies. In star-forming galaxies, the nebular metallicity Zneb is correlated with the light-weighted mean stellar age $\langle\,\log\,t_{\ast}\rangle_{\rm L}$ in an intermediate strength, and Zneb is weakly correlated with the mass-weighted mean stellar metallicity $\langle\,Z_{\ast}\rangle_{\rm M}$.
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