Before the Bar: The Hidden Bulge of the Milky Way

这项提议的目的是利用年老的、缺乏金属的恒星来追踪银河系中可能存在的经典凸起。我们相信，通过探测最古老的发光凸起恒星——天琴座RR星（RRLs），我们终于发现了长期寻找的压力支撑的凸起成分，我们建议使用光谱学，通过动力学、恒星化学成分和执行动力学模拟来了解它的起源。我们的试点调查显示，凸起rrl是已知的第一个凸起群，表现出热运动学，并具有零或可忽略的旋转，正如预期的经典凸起。因此，他们完全不同于突出的旋转条形/凸起型人口。它们的丰度分布比恒星光晕更富金属，但比凸起/条形人口的金属贫乏近1个指数。它们的恒星脉动特性有力地证明了它们与银河系的任何组成部分都不同，而且可能与M31有关。凸起的rrl与在银河系凸起中主导恒星质量的凸起巨星的运动学差异表明，银河系有一个复合凸起。因此，银河系凸起有两个形成时期：早期的时期保存在古老的rrl的运动学中，而后来的时期则是大质量的圆盘，已经演变成条形，经历了恒星形成的爆发。这些凸起的rrl可能是银河系中最古老的恒星，它们是银河系其他部分由内而外形成的核心。现在有足够的理由对这一新的恒星群进行更多的研究。详细的化学动力学分析将使我们了解银河系凸起处的这一古老恒星群，这一群恒星群的形成时间早于今天主导中心质量的“棒状星系”。

The aim of this proposal is to use old, metal-poor stars to trace a possible classical bulge in the Milky Way. We believe to have finally uncovered the long sought pressure-supported bulge component by probing the oldest luminous bulge stars, the RR Lyrae stars (RRLs), and we propose to use spectroscopy to understand its origin, via dynamics, stellar chemical compositions and by performing dynamical simulations. Our pilot survey has revealed that the bulge RRLs are the first bulge population known to exhibit hot kinematics and to have null or negligible rotation, as is expected for a classical bulge. They are therefore completely distinct from the prominent rotating bar/bulge population. Their abundance distribution is more metal rich than the stellar halo, yet nearly 1 dex more metal poor than the bulge/bar population. Their stellar pulsation properties argue powerfully for their being distinct to any Milky Way component and perhaps related to M31. The difference of the kinematics of the bulge RRLs from the bulge giants, which dominate the stellar mass in the Galactic bulge, demonstrates that the Milky Way has a composite bulge. Therefore, there were two formation epochs for the Galactic bulge: an early epoch that is preserved in the kinematics of the ancient RRLs, and a later epoch in which the massive disk, which has evolved into the bar, experienced a burst of star formation. These bulge RRLs may be the oldest stars in the Galaxy, serving as a nucleus around which the rest of the Galaxy was built up from the inside out. The case is now strong to undertake more investigation of this new stellar population. A detailed chemo-dynamic analysis will allow an understanding of this ancient stellar population in the Galactic bulge, which is a population that pre-dated the formation of the ``bar" that dominates the central mass in the present day.