Before the Bar: The Hidden Bulge of the Milky Way

这项计划的目的是利用古老的贫金属恒星来追踪银河系中可能存在的经典核球。我们相信，通过探测最古老的发光球型恒星RR天琴座恒星（RRL），我们终于发现了长期寻求的压力支持的球型成分，我们建议使用光谱学来了解其起源，通过动力学，恒星化学成分和进行动力学模拟。 我们的试点调查显示，隆起RRL是已知的第一个隆起人口表现出热运动学和具有零或可忽略的旋转，是一个经典的隆起。因此，它们完全不同于突出的旋转棒/凸起种群。它们的丰度分布比恒星晕更富金属，但比凸起/棒人口少近1倍的金属。 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。 星系核球RRL的运动学与星系核球巨星的运动学不同，后者在星系核球中占主导地位，这表明银河系有一个复合星系核球。因此，有两个形成时期的银河系隆起：早期的时代，是保存在古代RRL的运动学，和较晚的时代，其中大规模的磁盘，这已经演变成酒吧，经历了爆发的星星的形成。这些凸起的RRL可能是银河系中最古老的恒星，作为一个核心，银河系的其他部分从内到外围绕着它建立起来。现在有充分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能够了解银河系凸起中的这一古老恒星种群，这是一个在今天主导中心质量的"棒”形成之前的种群。

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.