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

majewskicold暗物质（CDM）宇宙结构增长模型预测，大型结构的增长来自较小结构的积累，以及大晕内长期存在的“亚晕”亚结构。然而，观测到的银河系卫星的数量比大多数CDM模型预测的要少几个数量级，而且，如果这些亚晕是暗的，它们的存在应该会动态地加热冷的螺旋盘、矮卫星和星团。因此，在完善CDM范式以解释像银河系这样的星系的演化方面，仍然存在相当大的问题。如果银河系晕是由发光的亚晕合并而成，我们期望在晕星、星团和卫星的相空间分布中看到亚结构。由于银河潮汐效应，这个亚结构将以来自被破坏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）矮球体和帕洛马5星团的恒星流一样。在过去的十年中，有一些诱人的证据表明，晕星的分布是非常块状的。如果合并的CDM亚晕是暗的，那么像Sgr和Pal 5这样的恒星流的相干性对亚晕的质谱有很强的限制。维吉尼亚大学的Steven Majewski和他的同事们将对这个“新”的银河系光晕有一个更全面的了解，他们将对目前的光晕形成模型设定基本边界条件的吸积亚结构进行大量的扩展。他们将确定潮汐碎片的实际填充系数和吸积实体的可能质谱。这些测量，反过来，允许推断银河系的质量分布和银河系的吸积历史。该计划自然地扩展了先前的工作，并从两个方向解决这些问题：(1)银河卫星系统-最有可能是先前吸积实体的原型的物体，其中包括目前对吸积晕的贡献者。首先，这个研究小组将对Sgr矮球状星系——银河系吸积的范例——进行全面的研究，利用由两微米全天观测提供的Sgr核心和潮汐尾的清晰全天视图。其次，他们将继续分析在许多银河系矮球体卫星周围发现的扩展密度剖面，并确定这些金剖面“破碎群”是代表束缚的恒星晕还是未束缚的潮汐碎片。最后，他们将把对银河卫星潮汐破坏的搜索扩大到晕球状星团系统，寻找类似于Pal 5的天体。(2)该小组将利用三个大的K巨星目录，在光环场恒星群中进行系统的亚结构搜索：即将完成的网格巨星调查，伴星深巨星调查（DGSS），以及在银河北极对K巨星进行225平方度的调查。在DGSS的试点版本中，研究小组在麦哲伦星云外围的区域进行了观测，并发现了大量证据，证明有一个由冷潮汐流高度连接的外晕，而没有明显的由热的、压力支持的晕星组成的贡献。重要的是要确定这一发现是否仅限于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的毕业论文研究项目，并协助收集数据，作为当地天文台半专用光谱项目的一部分。Majewski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，以加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.***