If the stellar halos of disk galaxies are built up from the disruption of dwarf galaxies, models predict highly structured variations in the stellar populations within these halos. We test this prediction by studying the ratio of blue horizontal branch stars (BHB stars; more abundant in old, metal-poor populations) to main-sequence turn-off stars (MSTO stars; a feature of all populations) in the stellar halo of the Milky Way using data from the Sloan Digital Sky Survey. We develop and apply an improved technique to select BHB stars using ugr color information alone, yielding a sample of ~9000 g 18 candidates where ~70% of them are BHB stars. We map the BHB/MSTO ratio across ~1/4 of the sky at the distance resolution permitted by the absolute magnitude distribution of MSTO stars. We find large variations of the BHB/MSTO star ratio in the stellar halo. Previously identified, stream-like halo structures have distinctive BHB/MSTO ratios, indicating different ages/metallicities. Some halo features, e.g., the low-latitude structure, appear to be almost completely devoid of BHB stars, whereas other structures appear to be rich in BHB stars. The Sagittarius tidal stream shows an apparent variation in the BHB/MSTO ratio along its extent, which we interpret in terms of population gradients within the progenitor dwarf galaxy. Our detection of coherent stellar population variations between different stellar halo substructures provides yet more support to cosmologically motivated models for stellar halo growth.
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机译：如果盘状星系的恒星晕是由矮星系的破坏建立的，则模型预测这些晕圈中的恒星种群具有高度结构化的变化。我们通过研究恒星晕中的蓝色水平分支恒星（BHB恒星;在较旧的，金属贫乏的种群中更丰富）与主序熄灭恒星（MSTO恒星;所有种群的特征）的比率，检验了这一预测。使用Sloan Digital Sky Survey的数据获得银河系。我们开发并应用了一种改进的技术来仅使用ugr颜色信息来选择BHB恒星，从而产生了约9000 g <18个候选样品的样本，其中约70％是BHB恒星。我们以MSTO恒星绝对量级分布所允许的距离分辨率绘制了BHB / MSTO比值覆盖了大约1/4的天空。我们发现恒星晕中BHB / MSTO恒星比有很大变化。先前发现的类流光晕结构具有独特的BHB / MSTO比，表明不同的年龄/金属。一些晕轮特征（例如低纬度结构）似乎几乎完全没有BHB星，而其他结构似乎富含BHB星。射手座潮汐流显示了BHB / MSTO比率沿其范围的明显变化，我们用祖先矮星系内的种群梯度来解释。我们对不同恒星晕亚结构之间的相干恒星种群变化的检测为恒星晕增长的宇宙学动机模型提供了更多支持。
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