Learning how the Milky Way was Assembled through Measurements of Positions, Velocities and Elements in Stars in the Halo

银河系是我们在宇宙中的家。银河系是我们理解周围宇宙中数千亿个星系最重要的参考。它是唯一一个我们可以详细观察单个恒星的星系，比如我们的太阳。然而，关于银河系和我们今天看到的结构，仍然有一些根本的问题没有得到解答。我们仍然不知道我们银河系的质量是否在两倍之内。对于星系如何形成，最被接受的模型是星系通过与较小的卫星星系合并而成长。研究人员将通过对银河系外部被称为光晕的古老恒星的光学观测来检验这一模型。利用光谱，研究人员将测量这些恒星中元素的丰度，这可以告诉我们这些古老恒星诞生的原始、较小的星系。结合对这些恒星的其他观测，研究人员将能够回答有关我们银河系历史的基本问题。银晕中恒星的深度光谱为这些恒星的自然运动增加了视线速度、化学丰度信息和改进的距离估计。研究人员的观测将确定速度的三个分量，每颗恒星在银河系中的位置的三个分量，以及恒星的关键第七维元素丰度。光谱和自行将测量远至90kpc的主序星。这一计划总共将获得450颗主序晕星的光谱，这些恒星在晕中的11条不同视线上。这些数据包含了关于银河系恒星晕增长和卫星吸积的独特信息，这些信息将被用来重建我们银河系的质量吸积史。此外，完整的相空间信息将被用来测量银河系的总质量；这是一个重要的、但缺乏约束的天体物理量。来自探测器自行数据的横向速度信息将被用来测量遥远晕星的速度各向异性。在我们对银河系总质量的了解中，速度各向异性是系统不确定性的主要来源。研究人员将在多维数据中搜索遥远光晕中的子结构，特别是来自低光度卫星的子结构。这些矮小星系的潮汐残留物在之前的搜索中被隐藏起来。测量低亮度晕前体的数量将测试暗物质亚晕和矮星星系之间的联系的模型。这是对宇宙星系形成模型预测的重要一致性检验。从速度的三个维度，研究人员将测量凝聚在光晕中的前驱系统的轨道。根据金属度，可以估算出前体的发光强度。阿尔法元素的丰度将被测量样本中较亮的恒星，并用于估计卫星的吸积时间。星体的轨道、吸积时间和亮度的分布决定了银河系晕的吸积史。他们的外展计划包括通过加州大学圣克鲁斯分校的科学实习计划(SIP)吸引高中生。高中SIP实习生与他们的导师在研究项目上合作，过去的实习生在天文学会的会议上提交过论文，并与人合著了被推荐的出版物。这两名研究生将与调查人员合作指导学生。这将给研究生提供非课堂教学的经验。研究生将为高中生开发一个基于Python的研究方法模块，该模块将从研究计划中吸取教训。调查人员将致力于增加未被充分代表的少数族裔在STEM领域的参与，方法是参与SIP、研究方法、加州州立数学和科学暑期学校以及圣克鲁斯县监狱的囚犯教育项目。

The Milky Way Galaxy is our home in the universe. The Milky Way is our most important reference for understanding the hundreds of billions of galaxies in the universe around us. It is the only galaxy where we can observe individual stars, like our sun, in detail. However, there remain fundamental unanswered questions about the Milky Way and the structure we see today. We still do not know the mass of our Galaxy to within a factor of two. The most accepted model for how galaxies form is that galaxies grow by merging with smaller satellite galaxies. The investigators will check this model by optical observations of ancient stars in the outer parts of the Milky Way, called the Halo. Using optical spectra, the investigators will measure the abundance of elements in these stars, which can tell us about the original, smaller, galaxies in which those old stars were born. When combined with other observations of these stars, the investigators will be able to answer basic questions about the history of our own Milky Way galaxy.The deep spectroscopy of stars in the galactic halo add line-of-sight velocity, chemical abundance information and improved distance estimates to the proper motions for these stars. The investigators' observations will determine the three components of the velocity, three components of each star's position in the Galaxy and a critical seventh dimension elemental abundances of the star. The spectroscopy and proper motions will be measured for main sequence stars as distant as 90 kpc. In total, this program will obtain spectra for 450 main sequence halo stars over 11 different lines of sight in the halo. These data contain unique information about the growth of the Milky Way's stellar halo and accretion of satellites, which will be used to reconstruct the mass accretion history of our Galaxy. Furthermore, the full phase-space information will be used to measure the total mass of the Milky Way; an important, yet poorly constrained, astrophysical quantity. The transverse velocity information from the investigator's proper motion data will be used to measure the velocity anisotropy of distant halo stars. The velocity anisotropy is a major source of systematic uncertainty in our knowledge of the total mass of the Milky Way. The investigators will search the multi-dimensional data for substructure in the distant halo, especially substructure from low luminosity satellites. The tidal remnants of these dwarf galaxies have been hidden from previous searches. Measuring the number of low-luminosity halo progenitors will test models for the connection between dark matter sub-halos and dwarf satellite galaxies. This is an important consistency test of the predictions of cosmological galaxy formation models. From the three dimensions of velocity, the investigators will measure the orbits of the progenitor systems that were accreted into the halo. From the metallicities, the luminosities of the progenitors can be estimated. The abundances of the alpha-elements will be measured for the brighter stars in the sample and used to estimate the accretion times of the satellites. The distributions of orbits, accretion times and luminosities of the progenitors defines the accretion history of our Galaxy's halo. Their outreach plan includes bringing in high school students via the UC Santa Cruz Science Internship Program (SIP). High school SIP interns work with their mentors on research projects, and past interns have presented papers at Astronomical Society meetings and have been co-authors on refereed publications. The two graduate students will mentor the students in collaboration with the investigators. This will give the graduate students experience in non-classroom teaching. Graduate students will develop a Python-based research methods module for high school students that draws from the research program. The investigators will work to increase the participation of underrepresented minorities in STEM fields through their participation in programs like SIP, Research Methods, the California State Summer School for Mathematics and Science and the Project for Inmate Education in the Santa Cruz county jail.