用近红外钙三重态从河外球状星团的积分光谱推算金属丰度
Deriving Metallicities From the Integrated Spectra of Extragalactic Globular Clusters Using the Near-Infrared Calcium Triplet
摘要：
近红外波段的Ca三重态（CaT）特征已被用作单个恒星以及银河系球状星团（GC）和星系的综合光的金属丰度指标，取得了不同程度的成功，有时也会产生令人困惑的结果。使用凯克的DEIMOS多目标摄谱仪，我们获得了144个完整的光谱样本的GC周围的最亮的星系NGC 1407来测试是否CaT指数可以作为一个金属丰度指标的河外GC。不同的单恒星星族模型对CaT作为金属丰度函数的行为做出了不同的预测。在这项工作中，使用经验转换从CaT指数值获得NGC 1407周围GC的金属丰度。测得的CaT/金属丰度分布显示出意想不到的功能，最显着的是，最明亮的红色和蓝色GC具有相似的CaT值，尽管他们的平均颜色差异很大。对NGC 1407 GC系统中这种行为的建议解释是：1）CaT可能受到热蓝色恒星的影响，2）CaT可能比模型预测的更早饱和，和/或3）颜色可能不是线性跟踪金属丰度。直到这些可能性被理解，使用的CaT作为金属丰度指标的综合光谱的河外GC将仍然存在问题。
英文摘要：
The Ca triplet (CaT) feature in the near-infrared has been employed as a metallicity indicator for individual stars as well as integrated light of Galactic globular clusters (GCs) and galaxies with varying degrees of success, and sometimes puzzling results. Using the DEIMOS multi-object spectrograph on Keck we obtain a sample of 144 integrated light spectra of GCs around the brightest group galaxy NGC 1407 to test whether the CaT index can be used as a metallicity indicator for extragalactic GCs. Different sets of single stellar population models make different predictions for the behavior of the CaT as a function of metallicity. In this work, the metallicities of the GCs around NGC 1407 are obtained from CaT index values using an empirical conversion. The measured CaT/metallicity distributions show unexpected features, the most remarkable being that the brightest red and blue GCs have similar CaT values despite their large difference in mean color. Suggested explanations for this behavior in the NGC 1407 GC system are: 1) the CaT may be affected by a population of hot blue stars, 2) the CaT may saturate earlier than predicted by the models, and/or 3) color may not trace metallicity linearly. Until these possibilities are understood, the use of the CaT as a metallicity indicator for the integrated spectra of extragalactic GCs will remain problematic.
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