The tilt of the velocity ellipsoid in the Milky Way disc
The tilt of the velocity ellipsoid in the Milky Way disc
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DOI：
10.1093/mnras/stv1314
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发表时间：
2014-07
影响因子：
4.8
通讯作者：
A. Budenbender;G. Ven;L. Watkins
中科院分区：
文献类型：
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作者：
A. Budenbender;G. Ven;L. Watkins
准确测定局部暗物质密度对于了解宇宙中暗物质的性质和分布具有重要意义。这就要求正确地刻画局部速度分布。本文对太阳附近的16,276颗SDSS/SEGUE G型矮星进行了运动学研究，确定了子午面上速度椭球的形状。我们根据G-矮星的[Fe/H]和[/Fe]丰度来分离它们，并使用考虑了可能的污染物的最大似然方法独立地估计每个子样本的最佳拟合银河系模型。我们证明，只有当我们允许圆盘中的速度椭球倾斜时，不同的子群才会产生一致的结果，从而证明了圆盘中径向运动和垂直运动解耦的常见假设是不正确的。此外，我们还发现速度椭球的倾角随着高度的增加而增加，从0.5kpc时的5.2增加到2.0kpc时的14.3，与指向银河系中心的角度tan()‘jzj=R一致。我们还证实了早期的发现，即亚群在径向和垂直方向上的行为几乎是等温的，对于化学年轻、富金属的盘星，大约39(20)公里的S 1到大约60公里的S 1(48公里S 1)对于化学老的、贫金属的盘星。我们的结论是，在考虑太阳近邻的动力学模型时，速度椭球倾斜所捕捉到的径向和垂直运动之间的耦合是不能忽略的。在接下来的论文中，我们将根据这些结果开发一个新的模拟方案，并改进局部暗物质密度的测定。
Accurate determination of the local dark matter density is important for understanding the nature and distribution of dark matter in the universe. This requires that the local velocity distribution is characterised correctly. Here, we present a kinematic study of 16,276 SDSS/SEGUE G-type dwarf stars in the solar neighbourhood, with which we determine the shape of the velocity ellipsoid in the meridional plane. We separate our G-dwarf stars based on their [Fe/H] and [ /Fe] abundances and estimate the best-fitting Milky Way model independently for each sub-sample using a maximum-likelihood method that accounts for possible contaminants. We show that the different subpopulations yield consistent results only when we allow the velocity ellipsoid in the disk to be tilted, demonstrating that the common assumption of decoupled radial and vertical motions in the disk is incorrect. Further, we the find that the tilt angle of the velocity ellipsoid increases with heightjzj from 5 2 at 0.5 kpc to 14 3 at 2.0 kpc, consistent with pointing toward the Galactic centre at an angle tan( )’ jzj=R. We also confirm earlier findings that the subpopulations behave almost isothermally both radially and vertically, about 39 (20) km s 1 for the chemically-young, metal-rich disk stars to about 60 km s 1 (48 km s 1 ) for the chemically-old, metal-poor disk stars. We conclude that the coupling between radial and vertical motion captured in the velocity ellipsoid tilt cannot be ignored when considering dynamical models of the solar neighbourhood. In a subsequent paper, we will develop a new modelling scheme informed by these results and make an improved determination of the local dark matter density.