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史。这项工作检验了吸积范式和双晕图景。如果光晕是通过许多矮小卫星星系的吸积和潮汐分裂在数十亿年内建立起来的，它将包含来自分裂星系的恒星流，这些恒星流可以作为恒星密度增强的区域(晕子结构)被检测到，也可以作为恒星一起在太空中移动。外晕可能主要由潮汐摧毁的卫星星系的恒星组成，而内晕可能是原地起源于吸积气体，或者是在合并形成银河系的最初物体内。人们认识到，该晕有两个主要成分，它们在恒星运动学、空间分布和金属丰度分布上不同。然而，只有一小部分光晕被调查过，一些已知的光晕亚结构的性质也没有得到很好的确定。光晕中不同的子结构和流可以通过天琴座RR恒星追踪到。这些脉动的恒星很容易通过它们的半天周期和大约一个星等的幅度来识别。这项研究将为之前探测到的晕子结构提供更好的距离和大小，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RR型天琴座恒星，距离太阳约60kpc。光谱观测被用来确定可疑的子结构是否是真正的共同运动的恒星群，就像预期的被吸积的卫星星系的残骸一样。测量了内晕中约150颗天琴座RR星的径向速度和金属度，以测试天琴座RR星是分裂成两个分量，还是集中在吸积晕中。该项目对研究生和本科生进行光学测光和光谱学技术培训。作为外展活动的一部分，将为耶鲁大学的莱特纳家庭天文台和天文馆制作一个关于银河系演化的节目。

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.