Explorations of the Milky Way's ``New'' Halo

AST0307851用于宇宙结构增长的Majewski冷暗物质(CDM)模型预测了大结构的增长，因为小结构的积累和大光晕中长期存在的“亚光晕”子结构的持续。然而，银河系观测到的卫星数量比大多数清洁发展机制模型预测的要少几个数量级，如果这些亚晕是暗的，它们的存在应该会动态加热冷螺旋盘、矮星和星团。因此，在完善CDM范式以解释像银河系这样的星系的演化过程中仍然存在相当大的问题。如果银河晕是由发光的亚晕合并而成，我们预计会在晕星、星团和卫星的相空间分布中看到子结构。由于银河系的潮汐效应，这个子结构将以来自分裂的母恒星系统的恒星流的形式出现，就像最近发现的来自人马座(SGR)矮球和Palomar 5星团的恒星流一样。过去十年来，越来越多的诱人证据表明，晕星的分布非常不均匀。如果合并的CDM子晕是暗的，像SGR和PAL 5这样的恒星流的相干性对次晕质谱造成了强烈的限制。弗吉尼亚大学的Steven Majewski和他的同事们将通过大幅扩展对吸积子结构的观测约束来更完整地了解这个“新的”银河系光晕，这些子结构为当前的光晕形成模型设定了基本的边界条件。他们将确定潮汐碎屑的实际填充系数和可能的吸积实体的质谱图。这些测量结果反过来又可以推断银河系的质量分布和银河系的吸积史。该计划自然地扩展了以前的工作，并从两个方向探讨了这些问题：(1)银河卫星系统--最有可能是以前被吸积实体的原型的物体，其中包括目前对被吸积晕做出贡献的物体。首先，这个研究小组将对SGR矮球星系--银河系吸积的范例--进行全面的研究，利用两个微米全天巡天计划提供的关于SGR核心和潮汐尾巴的清晰的全天图像。其次，他们将继续分析在一些银河系矮小球体卫星周围发现的扩展密度分布，并确定这些King分布的“破裂群体”代表的是结合的恒星晕还是非结合的潮汐碎片。最后，他们将通过搜索PAL 5类似物，将银河系卫星潮汐干扰的搜索扩展到光晕球状星系团系统。(2)该小组将利用三个大型K巨星星表系统地搜索晕场恒星群内的亚结构：即将完成的网格巨星调查、伴随的深巨星调查以及对北银河系极点K巨星的225平方度调查。在DGS的试验版本中，该团队在麦哲伦星云外围进行了观测，并发现了大量证据表明，外晕是由寒冷的潮汐流高度联网的，没有明显的贡献来自温度高、压力支持的晕星成分。重要的是要确定这一发现是否只针对麦哲伦星云附近的区域，或者它是否代表了光环的一般特征。该奖项的大部分资金是用于直接支持将大量参与研究工作的本科生和研究生。本科生的参与将包括个人高级论文研究项目和协助收集数据，作为当地天文台半专用光谱计划的一部分。马耶夫斯基博士将继续他积极的公开演讲承诺，并继续与弗吉尼亚大学数字媒体实验室合作，制作动画和其他图形，以增强对公众的银河结构科学的解释。*

AST 0307851MajewskiCold dark matter (CDM) models for the growth of structure in the Universe predict the growth of large structures from the accumulation of smaller ones and the persistence of long-lived, "sub-halo" substructure within large halos. However, the number of observed satellites of the Milky Way is orders of magnitude less than predicted by most CDM models, and, if these sub-halos are dark, their presence should heat dynamically cold spiral disks, dwarf satellites and star clusters. Thus, considerable problems remain in refining the CDM paradigm to explain the evolution of galaxies like the Milky Way. If the Milky Way halo formed from the merger of luminous sub-halos, we expect to see substructure in the phase space distribution of halo stars, clusters and satellites. Due to Galactic tidal effects, this substructure would be in the form of streams of stars from the disrupted parent star systems, like those recently discovered coming from the Sagittarius (Sgr) dwarf spheroidal and the star cluster Palomar 5. Tantalizing evidence that the distribution of halo stars is very lumpy has been accumulating over the past decade. If the merged CDM sub-halos were dark, the coherence of star streams like Sgr and Pal 5 place strong constraints on the sub-halo mass spectrum.Dr. Steven Majewski and colleagues at the University of Virginia will acquire a more complete understanding of this "new" Milky Way halo by substantially expanding observational constraints on accreted substructure which set essential boundary conditions on current halo formation models. They will determine the actual filling factor of tidal debris and the likely mass spectrum of accreted entities. These measurements, in turn, permit inference of the Galactic mass distribution and the accretion history of the Galaxy. The program naturally expands upon previous work and approaches these questions from two directions: (1) The Galactic satellite system - objects that are most likely to be the prototypes of previously accreted entities, among which are current contributors to the accreted halo. First, this research team will undertake a comprehensive study of the Sgr dwarf spheroidal galaxy - the paradigm for Milky Way accretion - exploiting the clear, all-sky view of the Sgr core and tidal tails provided by the Two Micron All-Sky Survey. Second, they will continue the analysis of extended density profiles found around a number of Galactic dwarf spheroidal satellites, and ascertain whether these King profile "break populations" represent bound stellar halos or unbound tidal debris. Finally, they will expand the search for tidal disruption of Galactic satellites to the halo globular cluster system with a search for Pal 5 analogues. (2) The group will undertake a systematic search for substructure within the halo field star population using three large catalogues of K giant stars: the almost completed Grid Giant Star Survey, the companion Deep Giant Star Survey (DGSS), and a 225 square deg survey of K giants at the North Galactic Pole. In a pilot version of the DGSS, the team have undertaken observations in fields around the Magellanic Cloud periphery and uncovered substantial evidence for an outer halo that is highly networked by cold tidal streams and with no apparent contribution by a hot, pressure-supported component of halo stars. It is important to determine whether this finding is specific to regions near the Magellanic Clouds, or whether it represents the general character of the halo.A majority of the funds in this award are for direct support of undergraduate and graduate students who will participate substantially in the research efforts. Undergraduate involvement will include individual Senior Thesis research projects and assistance with the gathering of data as part of a semi-dedicated spectroscopic program at the local observatory. Dr. Majewski will continue his active public lecture commitment, and continue to work with the University of Virginia Digital Media Lab to produce animations and other graphics that enhance the explanation of Galactic structure science to the public.***