[论文解读] Willman 1 - A Galactic Satellite at 40 kpc With Multiple Stellar Tails

Beth Willman, Morad Masjedi+7 more|arXiv (Cornell University)|Mar 17, 2006

Astronomy and Astrophysical Research被引用 12

一句话总结

本文展示了距离银河系40 kpc的遥远星系卫星Willman 1的广角测光数据，揭示其为一个古老、中等金属贫乏的系统，半光半径为21 ± 7 pc，绝对星等M_V = -2.5等。数据表明存在显著的多方向恒星尾迹和明显的质量分层现象，表明其受到银河系强烈的潮汐扰动，可能位于远星点附近或在过去的相互作用后发生，这挑战了球状星团与矮椭球星系之间的区分。

ABSTRACT

SDSSJ1049+5103, commonly known as Willman 1, is an extremely low-luminosity Milky Way companion whose properties are intermediate between those of globular clusters and dwarf spheroidals. In this paper, we use deep photometry to show that this object is old and moderately metal-poor, has a distance of 38 +/- 7 kpc, has an M_V of -2.5 mag, and has a half-light radius of 21 +/- 7 pc, consistent with previous estimates. The spatial distribution of Willman 1's main sequence stars shows 1) its total spatial extent exceeds its tidal radius for a range of assumptions about its total mass and its orbit and 2) the presence of prominent multi-directional stellar tails. The tidal interactions causing these tail features may explain the large physical size of Willman 1 relative to low-luminosity globular clusters. It is the most distant Galactic object yet known to display prominent tails, and is the only distant satellite to display multi-directional tails. Although we cannot at present determine the cause of this unusual morphology, preliminary comparisons between the morphology of Willman 1 and published simulations suggest that it may be near the apocenter of its orbit, or that it may have interacted with another halo object. We find a significant difference between the luminosity functions of stars in the center and in the tails of Willman 1, strongly suggesting mass segregation much like that seen in Palomar 5. Although Willman 1 has more pronounced tidal tails than most confirmed Milky Way dwarf galaxies, because of its very low stellar mass we cannot at present rule out the possibility that it has a dark matter halo. (Abridged)

研究动机与目标

• 利用深度广角测光，确定位于40 kpc处的低光度银河系卫星Willman 1的结构与运动学特性。
• 研究Willman 1是否处于动力学平衡状态，或是否正经历银河系的强烈潮汐扰动。
• 评估其多方向恒星尾迹的成因及其对轨道历史和质量光度比的启示。
• 评估观测到的质量分层现象是否支持其起源于球状星团，或暗示存在暗物质晕。

提出的方法

• 获取广角测光数据，覆盖主序转折点以下3个星等，以探测Willman 1的完整恒星种群。
• 通过恒星位置拟合与完备性修正，测量距离、半光半径和绝对星等。
• 分析主序星的空间分布，以识别多方向的潮汐尾迹。
• 比较中心区域与尾迹区域的光度函数，以检测质量分层现象。
• 基于动力学质量和轨道假设估算潮汐半径，评估系统的平衡状态。
• 与N体模拟进行对比，探索可能的轨道构型与相互作用历史。

实验结果

研究问题

• RQ1 Willman 1是否处于动力学平衡状态？其较大的空间尺度是否由银河系的强烈潮汐力引起？
• RQ2 Willman 1中观测到的多方向恒星尾迹由何引起？这对其轨道相位或过去相互作用意味着什么？
• RQ3 Willman 1中观测到的质量分层现象是否与动力学演化一致，或反映原始条件？其与Palomar 5等球状星团相比如何？
• RQ4 尽管恒星质量较低，Willman 1的形态与质量分层现象是否可与暗物质晕相容？抑或暗示其存在暗物质晕？
• RQ5 Willman 1是被破坏的球状星团还是矮椭球星系的可能性有多大？其形态如何为银河系恒星晕的形成提供启示？

主要发现

• Willman 1的修正V波段绝对星等为M_V = -2.5等，比先前估计更暗。
• 该系统的半光半径为21 ± 7 pc，距离为38 ± 7 kpc，确认其极为暗淡且位置遥远。
• 在多种质量与轨道假设下，Willman 1的总空间范围超过其潮汐半径，表明其受到银河系强烈的潮汐影响。
• 主序星的空间分布揭示出显著的多方向恒星尾迹，这是遥远银河系卫星中罕见的特征。
• 中心区域与尾迹区域的光度函数存在显著差异，为类似Palomar 5中观测到的质量分层现象提供了有力证据。
• 尽管恒星质量较低，该系统的形态与质量分层现象与暗物质晕一致，尽管此结论尚未得到证实。