Willman 1 -一个可能的矮星系，具有不规则的运动学分布
Willman 1 - a probable dwarf galaxy with an irregular kinematic distribution
摘要：
利用Keck/DEIMOS光谱和KPNO镶嵌相机成像技术研究了银河系卫星Willman 1（Wil 1）的运动学性质和恒星星族。Wil 1是一颗光度极低的银河系伴星。该天体位于大小光度空间（M_V ~ -2等，d ~ 38 kpc，r_half ~ 20 pc）的一个区域内，该区域也被银河系卫星牧夫座II和Segue 1和2占据，但没有其他已知的古老恒星系统。我们使用运动学和色星等的标准，以确定45星作为可能的成员WIL 1。Wil 1的系统速度为v_helio = -12.8 +/- 1.0 km/s，其速度与银河系前景恒星的速度相似。通过蒙特卡罗模拟，我们确定了45个候选成员星中的5个可能是前景污染物。我们确认了可能的Wil 1红巨星分支成员的丰度的显著分布（[Fe/H] = -1.73 +/- 0.12和-2.65 +/- 0.12，[Ca/Fe] = -0.4 +/- 0.18和+0.13 +/- 0.28）。这一分布支持了Wil 1是一个超低光度矮星系而不是星团的假设。Wil 1最内层的恒星以8 km/s的径向速度移动，速度色散与0 km/s一致，这表明Wil 1可能没有处于动力学平衡状态。前景污染和不寻常的运动学分布相结合，使得很难鲁棒地确定Wil 1的暗物质质量。因此，X射线或伽马射线观测的Wil 1，试图约束粒子暗物质的模型使用平衡质量模型强烈影响的系统在这里提出的意见。我们的结论是，尽管不寻常的功能在威尔1运动学分布，证据表明，这个对象是，或至少曾经是，一个矮星系。
英文摘要：
We investigate the kinematic properties and stellar population of the Galactic satellite Willman 1 (Wil 1) by combining Keck/DEIMOS spectroscopy with KPNO mosaic camera imaging. Wil 1 is an ultra-low luminosity Milky Way companion. This object lies in a region of size-luminosity space (M_V ~ -2 mag, d ~ 38 kpc, r_half ~ 20 pc) also occupied by the Galactic satellites Boötes II and Segue 1 and 2, but no other known old stellar system. We use kinematic and color-magnitude criteria to identify 45 stars as possible members of Wil 1. With a systemic velocity of v_helio = -12.8 +/- 1.0 km/s, Wil 1 stars have velocities similar to those of foreground Milky Way stars. Informed by Monte-Carlo simulations, we identify 5 of the 45 candidate member stars as likely foreground contaminants. We confirm a significant spread in the abundances of the likely Wil 1 red giant branch members ([Fe/H] = -1.73 +/- 0.12 and -2.65 +/- 0.12, [Ca/Fe] = -0.4 +/- 0.18 and +0.13 +/- 0.28). This spread supports the scenario that Wil 1 is an ultra-low luminosity dwarf galaxy rather than a star cluster. Wil 1's innermost stars move with radial velocities offset by 8 km/s from its outer stars and have a velocity dispersion consistent with 0 km/s, suggesting that Wil 1 may not be in dynamical equilibrium. The combination of the foreground contamination and unusual kinematic distribution make it difficult to robustly determine the dark matter mass of Wil 1. As a result, X-ray or gamma-ray observations of Wil 1 that attempt to constrain models of particle dark matter using an equilibrium mass model are strongly affected by the systematics in the observations presented here. We conclude that, despite the unusual features in the Wil 1 kinematic distribution, evidence indicates that this object is, or at least once was, a dwarf galaxy.
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