HST/ACS colour-magnitude diagrams of M 31 globular clusters
HST/ACS colour-magnitude diagrams of M 31 globular clusters
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DOI：
10.1051/0004-6361/200912587
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发表时间：
2009-10
影响因子：
6.5
通讯作者：
S. Perina;L. Federici;M. Bellazzini;C. Cacciari;F. F. Pecci-F.;S. G. D. D. Astronomia-S.-G.-D.-D.-Astronomia-102369218;U. Bologna;Italy.;I. A. D. Bologna
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文献类型：
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作者：
S. Perina;L. Federici;M. Bellazzini;C. Cacciari;F. F. Pecci-F.;S. G. D. D. Astronomia-S.-G.-D.-D.-Astronomia-102369218;U. Bologna;Italy.;I. A. D. Bologna
目标。为了增加M 31星团的样本，我们在HST档案中搜索了包含M 31球状星团修订博洛尼亚目录 * 中包含的天体的ACS图像。方法.我们利用ACS的图像来确认或修正它们的分类，并获得了11个年老球状星团和6个明亮年轻星团的有用CMD。我们获得了同时估计的距离，红化，和金属的老集群通过比较他们观察到的领域净化CMD与银河系的模板集群的网格。通过拟合理论等时线，我们估计了年轻星系团的年龄。结果对于旧的团簇，我们发现金属丰度范围为-0.4 ≤ [Fe/H] ≤-1.9。个人的估计一般同意现有的光谱估计。其中至少有四个显示一个清晰的蓝色水平分支，表明年龄在10 Gyr左右。发现所有六个候选年轻星团的年龄都小于1 Gyr。通过一个专门的网页公开提供星团的光度测定结果。结论.通过目前的工作，具有可靠光学CMD的老星系团的M31 GC总数从35个增加到44个，而年轻星系团则从7个增加到11个。老星系团表现出与MW星系团相似的特征。我们讨论的情况下，集群B407，金属丰度[Fe/H]<$-0.6，并位于一个大的投影距离的中心M31（Rp = 19.8 kpc）和星系的主轴（Y = 11.3 kpc）。富含金属的球状星团在大的银河系中心距离是罕见的，无论是在M 31和银河系。此外，B407的速度与大部分金属丰度相似的M31星团的旋转模式形成鲜明对比。这沿着其他的经验证据，支持了这样的假设，即星团（连同B403）在物理上与M31晕中的子结构有关，而M31晕被解释为合并事件的遗迹。
Aims. With the aim of increasing the sample of M 31 clusters for which a colour-magnitude diagram is available, we searched the HST archive for ACS images containing objects included in the Revised Bologna Catalogue of M 31 globular clusters***. Methods. Sixty-three such objects were found. We used the ACS images to confirm or revise their classification and were able to obtain useful CMDs for 11 old globular clusters and 6 luminous young clusters. We obtained simultaneous estimates of the distance, reddening, and metallicity of old clusters by comparing their observed field-decontaminated CMDs with a grid of template clusters of the Milky Way. We estimated the age of the young clusters by fitting with theoretical isochrones. Results. For the old clusters, we found metallicities in the range -0.4 ≤ [Fe/H] ≤ -1.9. The individual estimates generally agree with existing spectroscopic estimates. At least four of them display a clear blue horizontal branch, indicating ages ≳ 10 Gyr. All six candidate young clusters are found to have ages <1 Gyr. The photometry of the clusters is made publicly available through a dedicated web page. Conclusions. With the present work the total number of M 31 GCs with reliable optical CMD increases from 35 to 44 for the old clusters, and from 7 to 11 for the young ones. The old clusters show similar characteristics to those of the MW. We discuss the case of the cluster B407, with a metallicity [Fe/H] ≃ -0.6 and located at a large projected distance from the centre of M 31 (R p = 19.8 kpc) and from the major axis of the galaxy (Y = 11.3 kpc). Metal-rich globulars at large galactocentric distances are rare both in M 31 and in the Milky Way. B407, in addition, has a velocity in stark contrast with the rotation pattern shared by the bulk of M 31 clusters of similar metallicity. This, along with other empirical evidence, supports the hypothesis that the cluster (together with B403) is physically associated with a substructure in the halo of M 31 that has been interpreted as the relic of a merging event.