兴隆2.16米望远镜对M31晕中球状星团的光谱研究
Spectroscopic Study of Globular Clusters in the Halo of M31 with Xinglong 2.16m Telescope
摘要：
利用中国科学院国家天文台兴隆台2.16米望远镜OMR摄谱仪对11个已确认的M31球状星团进行了光谱观测。我们的9个样本星团位于M31的晕中，最远的一个距离银河系中心的投影半径为78.75 kpc。对于我们所有的样本集群，我们测量了舔吸收线指数和径向速度。结果表明，大部分GCs与M31星系的HI旋转曲线不同，特别是B514、MCGC 5、H12和B517，表明大部分GCs与星系的成星盘没有运动学关联。我们使用更新的恒星星族模型Thomas et al.（2010），通过应用$χ^2-$最小化方法，分别用Cassii和Padova的两条不同轨道拟合了吸收线指数。拟合结果表明，所有样品团簇的年龄均大于10 Gyr，且为贫金属（$-0.91 \le $ [Fe/H] $\le -2.38$ dex）。在将我们工作的光谱金属丰度与先前发表的光谱金属丰度合并后，我们将星系团样本扩展到距离星系中心117 kpc的投影半径。我们发现，所有确认的集群的金属丰度梯度存在的斜率为$-0.028\pm0.001$ dex kpc $^{-1}$。然而，对于所有的晕簇，斜率变为$-0.018\pm0.001$ dex kpc $^{-1}$，这要浅得多。如果我们只考虑具有$r_{\rm p}>25$ kpc的外晕团簇，则斜率变为$-0.010\pm0.002$ dex kpc $^{-1}$，如果从外晕样品中排除一个团簇G 001，则斜率为$-0.004\pm0.002$ dex kpc $^{-1}$。因此，我们认为M31外晕星系团的金属丰度梯度并不显著，这与前人的研究结果一致。
英文摘要：
We present the spectroscopic observations for 11 confirmed globular clusters of M31 with the OMR spectrograph on 2.16m telescope at Xinglong site of National Astronomical Observatories, Chinese Academy of Sciences. Nine of our sample clusters are located in the halo of M31 and the most remote one is out to a projected radius of 78.75 kpc from the galactic center. For all our sample clusters, we measured the Lick absorption-line indices and the radial velocities. It is noted that most GCs of our sample are distinct from the HI rotation curve of M31 galaxy, especially for B514, MCGC5, H12 and B517, suggesting that most of our sample clusters do not have kinematic association with the star forming young disk of the galaxy. We fitted the absorption line indices with the updated stellar population model Thomas et al. (2010) with two different tracks of Cassisi and Padova, separately, by applying the $χ^2-$minimization method. The fitting results show that all our sample clusters are older than 10 Gyr, and metal-poor ($-0.91 \le $ [Fe/H] $\le -2.38$ dex). After merging the spectroscopic metallicity of our work with the previously published ones, we extended the cluster sample out to a projected radius of 117 kpc from the galaxy's center. We found the metallicity gradient for all the confirmed clusters exists with a slope of $-0.028\pm0.001$ dex kpc$^{-1}$. However, the slope turns to be $-0.018\pm0.001$ dex kpc$^{-1}$ for all the halo clusters, which is much shallower. If we only consider the outer halo clusters with $r_{\rm p}>25$ kpc, the slope becomes $-0.010\pm0.002$ dex kpc$^{-1}$ and if one cluster G001 is excluded from the outer halo sample, the slope is $-0.004\pm0.002$ dex kpc$^{-1}$. Thus we conclude that metallicity gradient for M31 outer halo clusters is not significant, which agrees well with the previous findings.
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