Kinematics of Metal-poor Stars in the Galaxy. III. Formation of the Stellar Halo and Thick Disk as Revealed from a Large Sample of Nonkinematically Selected Stars
Kinematics of Metal-poor Stars in the Galaxy. III. Formation of the Stellar Halo and Thick Disk as Revealed from a Large Sample of Nonkinematically Selected Stars
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DOI：
10.1086/301409
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发表时间：
2000-03
期刊：
影响因子：
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通讯作者：
M. Chiba;T. Beers
中科院分区：
文献类型：
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作者：
M. Chiba;T. Beers
根据Beers等人最近修订和补充的无运动学偏差的贫金属星星表，对1203颗金属丰度[Fe/H] ≤-0.6的太阳邻域星的空间运动进行了详细分析。是迄今为止最大的此类目录。结果表明，[Fe/H] ≤-2.2的恒星可能代表“纯”晕成分，它们具有径向拉长的速度椭球（σU，σV，σW）=（141 ± 11，106 ± 9，94 ± 8）km s-1，小行星自转速度为30 ~ 50 km s-1，这与Beers和Sommer-Larsen先前仅基于径向速度信息对该样品的分析一致。与前面的分析相反，我们发现，对于可能是晕族成员的恒星，随着离银河系平面距离的增加，α V α V α V减小（Δ α V α V/Δ| Z| = -52 ± 6 km s-1 kpc-1），这可能代表了耗散形成的扁平内晕的特征。基本上与所有以前的运动学选择目录不同，我们样本中的贫金属星表现出不同的轨道偏心率分布，e，[Fe/H]和e之间没有明显的相关性。这清楚而令人信服地表明，埃根、林登-贝尔和桑德奇在1962年提出的银河系快速坍缩的证据，即晕星轨道偏心率与金属丰度之间的明显相关性，基本上是它们自行选择偏差的结果。然而，即使在我们的非运动学选择的样本中，我们也发现了一小部分浓度为[Fe/H]<$-1.7的高e星，这可能部分源于银河系早期形成期间的下沉气体。我们没有发现恒星丰度[Fe/H] ≤-2.2的额外厚盘成分的证据。靠近银道面的中等丰度恒星的运动学部分受到快速旋转的厚盘组件的影响，厚盘组件的速度为200 km/s（垂直速度梯度为Δ V/Δ）。|Z| = -30 ± 3 km s-1 kpc-1）和速度椭球（σU，σV，σW）=（46 ± 4，50 ± 4，35 ± 3）km s-1。在太阳附近的低金属量恒星中，当-2.2 < [Fe/H] ≤ -1.7时，属于厚盘星族的比例估计为10%;当-1.7 < [Fe/H] ≤-1时，属于厚盘星族的比例估计为30%。我们得到金属弱厚盘的径向尺度长度估计为4.5 ± 0.6kpc。我们还分析了构成银河系晕组成部分的恒星的全球运动学。晕的外部，我们认为是由轨道距离银道面超过5千秒差距的本地恒星所代表，没有系统自转。特别是，我们表明，以前的建议存在的“反旋转高晕”不支持我们的分析。外晕的密度分布接近球形，呈现幂律分布，精确的描述为ρ R-3.55±0.13。晕的内部是以旋光旋转和高度平坦的密度分布为特征的。我们没有发现明显的边界之间的内部和外部晕。我们证实了聚集在角动量相空间的少数地方金属贫恒星在1999年指出的Helmi等人。我们还确定了一个额外的拉长功能，在角动量相空间延伸的丛高方位角旋转的地区。检测到的簇中的成员数量与Helmi等人报道的相比没有显著增加，尽管我们考虑的样本恒星的总数几乎是以前调查的三倍。我们的结论是，晕星的分数，可能已经出现了从这个集群的前身对象可能小于10%的目前的银河系晕，如前所述。我们的研究结果的银河系的形成的影响进行了讨论，特别是在目前青睐的冷暗物质理论的层次星系形成的背景下。
We present a detailed analysis of the space motions of 1203 solar-neighborhood stars with metal abundances [Fe/H] ≤ -0.6, on the basis of a catalog, of metal-poor stars selected without kinematic bias recently revised and supplemented by Beers et al. This sample, having available proper motions, radial velocities, and distance estimates for stars with a wide range of metal abundances, is by far the largest such catalog to be assembled to date. We show that the stars in our sample with [Fe/H] ≤-2.2, which likely represent a "pure" halo component, are characterized by a radially elongated velocity ellipsoid (σU, σV, σW) = (141 ± 11, 106 ± 9, 94 ± 8) km s-1 and small prograde rotation ⟨Vϕ⟩ = 30 to 50 km s-1, consistent with previous analysis of this sample by Beers and Sommer-Larsen based on radial velocity information alone. In contrast to the previous analysis, we find a decrease in ⟨Vϕ⟩ with increasing distance from the Galactic plane for stars that are likely to be members of the halo population (Δ⟨Vϕ⟩/Δ|Z| = -52 ± 6 km s-1 kpc-1), which may represent the signature of a dissipatively formed flattened inner halo. Unlike essentially all previous kinematically selected catalogs, the metal-poor stars in our sample exhibit a diverse distribution of orbital eccentricities, e, with no apparent correlation between [Fe/H] and e. This demonstrates, clearly and convincingly, that the evidence offered in 1962 by Eggen, Lynden-Bell, & Sandage for a rapid collapse of the Galaxy, an apparent correlation between the orbital eccentricity of halo stars with metallicity, is basically the result of their proper-motion selection bias. However, even in our nonkinematically selected sample, we have identified a small concentration of high-e stars at [Fe/H] ∼ -1.7, which may originate, in part, from infalling gas during the early formation of the Galaxy. We find no evidence for an additional thick disk component for stellar abundances [Fe/H] ≤ -2.2. The kinematics of the intermediate-abundance stars close to the Galactic plane are, in part, affected by the presence of a rapidly rotating thick disk component with ⟨Vϕ⟩ ≃200 km s-1 (with a vertical velocity gradient on the order of Δ⟨Vϕ⟩/Δ|Z| = -30 ± 3 km s-1 kpc-1) and velocity ellipsoid (σU, σV, σW) = (46 ± 4, 50 ± 4, 35 ± 3) km s-1. The fraction of low-metallicity stars in the solar neighborhood that are members of the thick disk population is estimated as ∼10% for -2.2 < [Fe/H] ≤ -1.7 and ∼30% for -1.7 < [Fe/H] ≤ -1. We obtain an estimate of the radial scale length of the metal-weak thick disk of 4.5 ± 0.6 kpc. We also analyze the global kinematics of the stars constituting the halo component of the Galaxy. The outer part of the halo, which we take to be represented by local stars on orbits reaching more than 5 kpc from the Galactic plane, exhibits no systematic rotation. In particular, we show that previous suggestions of the presence of a "counter-rotating high halo" are not supported by our analysis. The density distribution of the outer halo is nearly spherical and exhibits a power-law profile that is accurately described as ρ ∝ R-3.55±0.13. The inner part of the halo is characterized by a prograde rotation and a highly flattened density distribution. We find no distinct boundary between the inner and outer halo. We confirm the clumping in angular-momentum phase space of a small number of local metal-poor stars noted in 1999 by Helmi et al. We also identify an additional elongated feature in angular-momentum phase space extending from the clump to regions with high azimuthal rotation. The number of members in the detected clump is not significantly increased from that reported by Helmi et al., even though the total number of the sample stars we consider is almost triple that of the previous investigation. We conclude that the fraction of halo stars that may have arisen from the precursor object of this clump may be smaller than 10% of the present Galactic halo, as previously suggested. The implications of our results for the formation of the Galaxy are discussed, in particular in the context of the currently favored cold dark matter theory of hierarchical galaxy formation.