Measurement of Halo Substructure by RR Lyrae Star Surveys

津恩博士和他的团队研究了银河系光晕的吸积历史。这项工作测试了吸积范式和双晕图。如果这个光晕是在数十亿年的时间里通过许多矮卫星星系的吸积和潮汐破坏而形成的，那么它将包含来自被破坏星系的恒星流，这些恒星流可以作为恒星密度增强的区域（光晕亚结构）和恒星在太空中一起运动的区域被探测到。外晕可能主要由来自被潮汐摧毁的卫星星系的恒星组成，而内晕可能起源于吸积气体或最初合并形成银河系的物体。我们认识到光晕有两个主要组成部分，它们在恒星运动学、空间分布和金属丰度分布上有所不同。然而，仅对光晕的一小部分进行了调查，并且一些已知的光晕子结构的性质也很难确定。光晕中不同的子结构和流可以用天琴座RR星来追踪。这些脉动恒星很容易通过它们的半日周期和大约一个星等的幅度来识别。这项研究将提供先前探测到的光晕子结构的更好的距离和大小，并在尚未调查的天空区域寻找新的子结构。这项工作使用了在LaSilla-QUEST调查中发现的距离太阳60kpc的RR天琴座恒星。光谱观测被用来确定可疑的亚结构是否真的是共同运动的恒星群，就像对吸积卫星星系残骸的期望一样。测量了内晕中大约150颗RR天琴座恒星的径向速度和金属丰度，以测试RR天琴座恒星是分成两部分，还是集中在吸积的晕中。该项目训练研究生和本科生光学光度学和光谱学技术。作为拓展活动的一部分，耶鲁大学莱特纳家族天文台和天文馆将举办一场关于银河系演化的展览。

Dr. Zinn and his team study the accretion history of the halo of the Milky Way galaxy. This work tests the accretion paradigm and the dual halo picture. If the halo was built up over billions of years through the accretion and tidal disruption of many dwarf satellite galaxies, it will contain streams of stars from disrupted galaxies that are detectable as regions of enhanced stellar density (halo substructures) and as stars moving together through space. The outer halo may consist primarily of stars from tidally destroyed satellite galaxies, whereas the inner halo may have originated in situ from accreted gas or within the very first objects that merged to form the Galaxy. It is recognized that the halo has two major components that differ in stellar kinematics, spatial distribution, and metallicity distribution. However only a small fraction of the halo has been surveyed, and properties of some known halo substructures are poorly determined. The different sub-structures and streams in the halo are traceable with RR Lyrae stars. These pulsating stars are easily recognized by their half-day periods and about one magnitude amplitudes. This research will provide better distances and sizes of previously detected halo substructures and look for new substructures in yet to be surveyed areas of the sky. This work uses RR Lyrae stars found in the LaSilla-QUEST survey up to about 60 kpc from the Sun. Spectroscopic observations are used to identify if suspected substructures are real groups of co-moving stars, as expected for remains of accreted satellite galaxies. Radial velocities and metallicities are measured for about 150 RR Lyrae stars in the inner halo to test whether RR Lyrae stars separate into two components, or are instead concentrated in the accreted halo. This project trains graduate and undergraduate students in the techniques of optical photometry and spectroscopy. As part of the outreach activities, a show on the evolution of the Milky Way galaxy will be created for Yale's Leitner Family Observatory and Planetarium.