Complexity on Small Scales: The Metallicity Distribution of the Carina Dwarf Spheroidal Galaxy
Complexity on Small Scales: The Metallicity Distribution of the Carina Dwarf Spheroidal Galaxy
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小尺度上的复杂性：船底座矮球状星系的金属丰度分布
DOI：
10.1086/499490
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发表时间：
2005
期刊：
影响因子：
--
通讯作者：
N. Evans
中科院分区：
文献类型：
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作者：
A. Koch;E. Grebel;R. Wyse;J. Kleyna;M. Wilkinson;D. Harbeck;G. Gilmore;N. Evans
船底座矮星球状星系是这类星系中唯一一个清楚地显示出恒星形成的间歇性的星系，它们之间有很长的停顿。在这里，我们给出了这颗银河卫星437个径向速度成员的金属度。作为智利欧洲南方天文台甚大望远镜大型项目的一部分，测量了金属度和径向速度。我们用多目标光谱仪火焰获得了中等分辨率的光谱。我们的目标红巨星覆盖了船底座的整个投影表面积。我们的光谱集中在近红外的CaII三重态，这是一个公认的老年和中年红巨星的金属丰度指标。由此得到的数据样本提供了迄今为止最大的局域群矮小球状星系的光谱金属含量集合。我们船底座的四个可能的径向速度成员位于这个星系标称的潮汐半径之外，支持了早先关于船底座主体以外可能存在这样的恒星的说法。在Carretta和Gratton的1997年金属丰度标度上，我们发现隆突的平均金属丰度为[Fe/H]∼-1.7dex。金属度分布函数的形式半高宽为0.92Dex，而整个金属度范围约为-3.0Dex<；[Fe/H]<；0.0Dex。金属丰度分布函数可能代表不同金属丰度的几个亚群。似乎有一个温和的径向梯度，使得更多富含金属的人群更加集中，这与越来越多的中年恒星的类似趋势相一致(参见Harbeck及其同事2001年的工作)。这一点，以及更富金属的红巨星的光度学颜色，表明船底座表现出年龄与金属丰度的关系。事实上，年龄-金属丰度的简并似乎共同形成了一个狭窄的红色巨型分支，尽管在金属丰度和广泛的年龄范围内有相当大的分布。金属丰度分布函数与简单的化学演化闭箱模型不能很好地匹配。通过同时考虑流入和流出的化学模型，可以获得定性上更好的匹配。对于所有这些模型来说，G矮星问题仍然存在。
The Carina dwarf spheroidal galaxy is the only galaxy of this type that shows clearly episodic star formation separated by long pauses. Here we present metallicities for 437 radial velocity members of this Galactic satellite. The metallicities and radial velocities were measured as part of a Large Programme with the Very Large Telescope at the European Southern Observatory, Chile. We obtained medium-resolution spectroscopy with the multiobject spectrograph FLAMES. Our target red giants cover the entire projected surface area of Carina. Our spectra are centered at the near-infrared Ca II triplet, which is a well-established metallicity indicator for old and intermediate-age red giants. The resulting data sample provides the largest collection of spectroscopically derived metallicities for a Local Group dwarf spheroidal galaxy to date. Four of our likely radial velocity members of Carina lie outside this galaxy's nominal tidal radius, supporting earlier claims of the possible existence of such stars beyond the main body of Carina. We find a mean metallicity of [Fe/H] ∼ -1.7 dex on the 1997 metallicity scale of Carretta and Gratton for Carina. The formal FWHM of the metallicity distribution function is 0.92 dex, while the full range of metallicities is found to span approximately -3.0 dex < [Fe/H] < 0.0 dex. The metallicity distribution function might be indicative of several subpopulations distinct in metallicity. There appears to be a mild radial gradient such that more metal-rich populations are more centrally concentrated, matching a similar trend for an increasing fraction of intermediate-age stars (see the 2001 work of Harbeck and coworkers). This, as well as the photometric colors of the more metal-rich red giants, suggests that Carina exhibits an age-metallicity relation. Indeed, the age-metallicity degeneracy seems to conspire to form a narrow red giant branch despite the considerable spread in metallicity and wide range of ages. The metallicity distribution function is not well matched by a simple closed-box model of chemical evolution. Qualitatively better matches are obtained by chemical models that also take into account infall and outflows. A G dwarf problem remains for all these models.