[论文解读] Omega Centauri: Nucleus of a Milky Way Dwarf Spheroidal?

Steven R. Majewski, Richard J. Patterson+5 more|arXiv (Cornell University)|Oct 14, 1999

Stellar, planetary, and galactic studies被引用 5

一句话总结

该论文基于其异常的金属度分布函数（MDF）和强烈逆行、与银盘共面的轨道，提出Omega Centauri 是银河系矮球状星系解体后的残余核心，而非典型的球状星团。该模型通过银河系银盘的潮汐剥离解释了其宽广的MDF（峰值在[Fe/H] = -1.7，延伸至更低丰度）和轨道动力学，对厚盘形成具有启示意义。

ABSTRACT

We focus on two aspects of Omega Cen that are rather uncharacteristic of globular clusters: its metallicity distribution and its orbit. We use (1) a Washington (M-T_2,M)_o color-magnitude diagram containing 130,000 stars, (2) DDO51 photometry to separate out giants from field dwarfs, and (3) a mapping of (M-T_2,M-DDO51)_o colors to [Fe/H] to derive a metallicity distribution function (MDF) for Omega Cen. The MDF is corrected for field giant contamination by a radial velocity membership study of a subsample of stars spanning all abundances. As with previous studies, the MDF is very wide with a peak near [Fe/H]=-1.7 and extending to both higher and lower abundance, but with few stars having [Fe/H] >~ -1.2. The orbit of Omega Cen is strongly retrograde, has small apogalacticon and is almost coplanar with the Milky Way disk -- i.e., an orbit unlike any known for almost all Galactic globular clusters, and unique among clusters with its horizontal branch type and [Fe/H]. We propose that the MDF and orbit of Omega Cen, when considered in light of other unusual Omega Cen properties are consistent with a picture wherein this object is the remains (nucleus) of a once larger satellite dwarf galaxy that has been substantially reduced by tidal stripping. A model wherein Omega Cen faced considerable stripping by the Galactic disk can account for the cluster's current state. A present-day counterpart of this process may be the Sagittarius dSph system, for which the globular cluster M54 (which has a mass similar to Omega Cen) has been purported to be the nucleus. A larger proto-Omega Cen on a retrograde orbit plowing through the Milky Way disk may have aided thick disk formation both through the contribution of tidally stripped stars and through substantial thin disk heating.

研究动机与目标

• 调查Omega Centauri为何表现出与典型球状星团不符的特性，特别是其宽广的金属度分布和异常轨道。
• 确定观测到的金属度分布和轨道特征是否能通过星系核心起源而非标准星团形成情景来解释。
• 评估银河系银盘的潮汐剥离在塑造Omega Cen当前结构和运动学中的作用。
• 探讨Omega Cen与银河系厚盘形成之间通过恒星剥离的潜在联系。

提出的方法

• 利用Omega Cen中130,000颗恒星构建高时间分辨率的华盛顿（M-T₂, M）₀颜色-星等图。
• 应用DDO51测光法以区分红巨星与场星矮星的污染。
• 将（M-T₂, M-DDO51）₀颜色映射至[Fe/H]，以推导金属度分布函数（MDF）。
• 利用径向速度成员数据对全丰度范围的场星红巨星污染进行MDF校正。
• 分析Omega Cen的轨道参数，重点关注其逆行运动、低远银心距及与银河系银盘的共面性。
• 模拟银河系银盘对前体矮球状星系的潮汐剥离效应，并与当前Omega Cen的特性进行比较。

实验结果

研究问题

• RQ1 Omega Centauri的宽广金属度分布是否能通过星系核心起源而非标准星团形成机制来解释？
• RQ2 为何Omega Centauri具有逆行轨道、低远银心距及与银盘共面的特性——这些特征与其他球状星团截然不同？
• RQ3 银河系银盘的潮汐剥离在多大程度上可解释Omega Cen的当前状态？
• RQ4 Omega Cen在动力学和化学特性上是否与一个解体的矮球状星系一致？
• RQ5 此类前体的潮汐碎屑是否可能贡献于银河系厚盘的形成？

主要发现

• Omega Centauri的金属度分布函数（MDF）极为宽广，峰值位于[Fe/H] = -1.7，且向更高和更低金属度延伸，高于[Fe/H] ≈ -1.2的恒星极少。
• Omega Centauri的轨道为强烈逆行，远银心距较小，且几乎与银河系银盘共面——这与其他银河系球状星团的轨道特性截然不同。
• 宽广的MDF与独特轨道的结合，与标准球状星团形成机制不一致，但与该天体为解体矮球状星系残余核心的假设一致。
• 银河系银盘的潮汐剥离很可能减少了前体系统的原始质量，从而解释了Omega Cen当前的致密且大质量状态。
• 该模型表明，前体系统可能通过恒星剥离和盘面加热，对银河系厚盘的形成有所贡献。
• 现今的类比可能是人马座矮球状星系（Sagittarius dSph），其中球状星团M54被提出为类似解体系统的残余核心。