Deep ACS Imaging of the Halo of NGC 5128: Reaching the Horizontal Branch
Deep ACS Imaging of the Halo of NGC 5128: Reaching the Horizontal Branch
复制标题
NGC 5128 光环的深度 ACS 成像：到达水平分支
DOI：
10.1086/432462
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发表时间：
2005
期刊：
影响因子：
--
通讯作者：
E. Peng
中科院分区：
文献类型：
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作者：
M. Rejkuba;L. Greggio;W. Harris;G. Harris;E. Peng
利用哈勃空间望远镜（HST）的宽视场照相机（WFC）对NGC 5128的外晕场进行了深（V，I）测光，观测到了极限星等I ~（129）。我们的测光首次直接揭示了巨大E/S 0星系中核心氦燃烧的恒星群体（“红团”或水平分支）。色星等图显示了一个非常宽的红巨星分支（RGB）、一个渐近巨星分支（AGB）隆起和红色团块;没有明显的蓝色水平分支星，证实了以前的建议，即在这个星系的晕中，古老的、非常贫金属的星群并不普遍。从上面的RGB，我们得到的金属丰度分布，我们发现这是非常广泛的和适度的金属丰富，平均[M/H] = -0.64和0.49 dex的色散。金属丰度分布函数与以前用HST观察到的其他晕场几乎相同，但由于这些非常红的恒星的V带不完整性，在以前的调查中部分错过了增强的富金属人口。结合RGB星的金属敏感颜色与AGB凸点和红团的金属敏感和年龄敏感特征，我们推断晕星的平均年龄为~8 Gyr。作为我们研究的一部分，我们提出了ACS F606 W和F814 W滤波器的标准V和I波段的经验校准，实现了从ESO拉西拉天文台的新技术望远镜的EMMI相机的同一领域的地面观测。
Using the Hubble Space Telescope (HST) Wide Field Camera (WFC) of the Advanced Camera for Surveys (ACS), we have obtained deep (V, I) photometry of an outer halo field in NGC 5128, to a limiting magnitude of I ≃ 29. Our photometry directly reveals the core helium burning stellar population (the "red clump" or horizontal branch) in a giant E/S0 galaxy for the first time. The color-magnitude diagram displays a very wide red giant branch (RGB), an asymptotic giant branch (AGB) bump, and the red clump; no noticeable population of blue horizontal branch stars is present, confirming previous suggestions that old, very metal-poor population is not ubiquitous in the halo of this galaxy. From the upper RGB we derive the metallicity distribution, which we find to be very broad and moderately metal-rich, with average [M/H] = -0.64 and dispersion 0.49 dex. The metallicity distribution function is virtually identical to that found in other halo fields observed previously with HST, but with an enhanced metal-rich population that was partially missed in the previous surveys due to V-band incompleteness for these very red stars. Combining the metallicity-sensitive colors of the RGB stars with the metallicity- and age-sensitive features of the AGB bump and the red clump, we infer the average age of the halo stars to be ~8 Gyr. As part of our study, we present an empirical calibration of the ACS F606W and F814W filters to the standard V and I bands, achieved with ground-based observations of the same field made from the EMMI camera of the New Technology Telescope of the ESO La Silla Observatory.