Stellar Populations of a Sample of Optically Selected AGN-host Dwarf Galaxies

In this paper we present our studies on the stellar populations and star formation histories (SFHs) for the Reines et al. sample of 136 dwarf galaxies which host active galactic nuclei (AGNs), selected from the Sloan Digital Sky Survey Data Release 8. We derive stellar populations and reconstruct SFHs for these AGN-host dwarfs using the stellar population synthesis code STARLIGHT. Our results suggest that these AGN-host dwarfs have assembled their stellar masses within a narrow period of time with the stellar mass-weighted ages in the range of $10^9-10^{10}$yr, but show a wide diversity of SFHs with the luminosity-weighted stellar ages in the range of $10^7-10^{10}$yr. The old population ($t>10^9$yr) contributes most to the galaxy light for the majority of the sample; the young population ($t<10^8$yr) also appears in significant but widely varying fractions, while the intermediate-age population ($10^8<t<10^9$yr) in general contributes less to the optical continuum at 4020 $\r{A}$. We also find that these dwarfs follow a similar mass-metallicity relation to normal star-forming galaxies, indicating that AGNs have little effect on the chemical evolution of the host galaxy. We further investigate the relation between the derived SFHs and morphology of the host galaxy, and find no correlation. Comparing the SFHs with the luminosity of the [OIII] $\lambda$5007 line ($L_{\rm [OIII]}$), we find that there exists a mild correlation when $L_{\rm [OIII]} > 10^{39}$erg s$^{-1}$, indicating that there is a physical connection between star formation and AGN activities in these dwarf galaxies.