早型星系中中等年龄恒星星族的K波段光谱证据
Evidence for intermediate-age stellar populations in early-type galaxies from K-band spectroscopy
摘要：
对不同环境下早型星系中恒星星族的研究是制约其恒星形成历史的有力工具。这项研究传统上仅限于光学范围，其中关闭周围的矮星和RGB底部的恒星在所有年龄段都占主导地位。近红外光谱范围是特别有趣的，因为在存在一个中等年龄的人口，AGB星是主要的贡献者。在这封信中，我们测量了低密度环境中12个早型星系样本的近红外指数NaI和D $_{\rm CO}$，并将它们与Silva等人（2008）提出的天炉座星系样本进行了比较。结合光学范围内的Lick/IDS指数对这些指数的分析表明：i）NaI指数在光学范围内是与C4668一样好的金属丰度指标; ii）D $_{\rm CO}$是中等年龄恒星星族的示踪物。我们发现低密度环境中的低质量星系比位于天炉座星系团中的低质量星系具有更高的NaI和D $_{\rm CO}$，这表明在低密度环境中的星系处于恒星形成的后期阶段，与其他使用独立方法的研究结果一致。
英文摘要：
The study of stellar populations in early-type galaxies in different environments is a powerful tool for constraining their star formation histories. This study has been traditionally restricted to the optical range, where dwarfs around the turn-off and stars at the base of the RGB dominate the integrated light at all ages. The near-infrared spectral range is especially interesting since in the presence of an intermediate-age population, AGB stars are the main contributors. In this letter, we measure the near-infrared indices NaI and D$_{\rm CO}$ for a sample of 12 early-type galaxies in low density environments and compare them with the Fornax galaxy sample presented by Silva et al. (2008). The analysis of these indices in combination with Lick/IDS indices in the optical range reveals i) the NaI index is a metallicity indicator as good as C4668 in the optical range, and ii) D$_{\rm CO}$ is a tracer of intermediate-age stellar populations. We find that low-mass galaxies in low density environments show higher NaI and D$_{\rm CO}$ than those located in Fornax cluster, which points towards a late stage of star formation for the galaxies in less dense environments, in agreement with results from other studies using independent methods.
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