| 其他摘要 | The co-evolution of galaxies and supermassive black holes (SMBHs) in the center of galaxies is a remarkable discovery in modern astronomy. In observations and numerical simulations, more and more evidences suggest that active galactic nuclei (AGN) have significant effects on the star formation of massive host galaxies. However, the importance of AGN in regulating the star formation of dwarf galaxies is still in debate. AGN may be triggered by the gas inflows into the nuclear or mass loss from evolving stars. Meanwhile, the gas may also be used to form stars. Therefore, the stellar popula- tions of AGN-host galaxies are an important tool for studying the connection between AGNs and star formation. In this paper, we present a detailed study of the stellar popu- lations of AGN-host dwarf galaxies and try to investigate the relation between nuclear activities and the star formation histories (SFHs) of AGN-host dwarfs. 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 109 − 1010 yr, but show a wide diversity of SFHs with the luminosity-weighted stellar ages in the range of 107 − 1010 yr. The old population (t > 109 yr) contributes most to the galaxy light for the majority of the sample; the young population (t < 108 yr) also appears in significant but widely varying fractions, while the intermediate-age population (108 < t < 109 yr) in general contributes less to the optical continuum at 4020 Å. 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 [O III] λ5007 line (L[OIII]), we find that there exists a mild correlation when L[OIII] > 1039 erg s−1, indicating that there is a physical connection between star formation and AGN activities in these dwarf galaxies. The reason for the connection might be caused by that the gas entering the center of the galaxy can not only be used to form stars, but also feed the supermassive black hole at the same time. Based on MaNGA integral field unit (IFU) spectroscopy we search 60 AGN candi- dates, which have stellar masses M⋆ ⩽ 5 × 109 M⊙ and show AGN ionization signatures in the BPT diagram. For these AGN candidates, we derive the spatially resolved stellar population with the stellar population synthesis code STARLIGHT and measure the gradients of the mean stellar age and metallicity. We find that the gradients of mean stellar age (metallicity) of individual AGN-host dwarfs are diverse in 0-0.5 Re, 0.5-1 Re and 0-1 Re. However, the overall behavior of the mean stellar age (metallicity) profiles tend to be flat, as the median values of the gradients are close to zero. We further study the overall behavior of the mean stellar age (metallicity) by plotting the co-added radial profiles for the AGN sample and compare with a control sample with similar stellar mass. We find that the median values of light-weighted mean stellar ages of AGN sample are as old as 2−3 Gyr within 2 Re,which are about 4−7 times older than those of the control sample. Meanwhile, most of the AGN candidates are low-level AGNs, as only eight sources have L[OIII]>1039.5 erg s−1. Hence, the AGNs in dwarf galaxies might accelerate the evolution of galaxies by accelerating the consumption of the gas, resulting in an overall quenching of the dwarf galaxies, and the AGNs also become weak due to the lack of gas. The median values of mass-weighted mean stellar age of both samples within 2 Re are similar and as old as about 10 Gyr, indicating that the stellar mass is mainly contributed by old stellar populations. The gradients of co-added mean stellar metallicity for both samples tend to be negative but close to zero, and the similar mean stellar metallicity profiles for both samples indicate that the chemical evolution of the host galaxy is not strongly influenced by the AGN. |
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