Weighing and measuring the Milky Way and other galaxies

罗宾·桑德森博士被授予美国国家科学基金会天文学和天体物理学博士后奖学金，在哥伦比亚大学和纽约大学开展研究和教育项目。在包括太阳在内的大多数恒星所在的银河系盘外，我们银河系的大部分质量都在它的暗物质光晕中，暗物质是一种看不见的物质，只有通过引力影响才能存在。另外还有几颗晕星，包括进入银河系暗晕的较小星系的散落残骸，结果被潮汐力量撕裂。留下的结构被称为潮汐流，可以用来研究银河系暗物质的分布。每个潮汐流都是由曾经是同一个小星系中的邻居的恒星组成的，因此现在它们围绕银河系运行时遵循的是相似的路径。这个共同的起源让我们深入了解了银河系的质量分布，这决定了星系流中恒星的轨道：只有当我们猜测接近真实的质量分布时，每个星系流中恒星的计算轨道才会聚集在一起，围绕着一个共同的祖先。潮汐流星邻近轨道的变化也可能为较小尺度的暗物质结构提供证据：一些暗物质理论预测，微小的暗物质团块太小，无法容纳星系，将以可预测的方式偏离流星的路径。桑德森博士将开发新的工具，通过研究暗物质晕的潮汐流来探索暗物质晕的结构，特别是银河系的暗物质晕。这些工具使用最先进的方法，但基于孩子们在中学时学到的基本概念：为什么星星会发光，我们如何制作和阅读地图，以及重力是如何工作的。为了教授这些核心思想，激发中学生对科学的兴趣，桑德森博士将开发便携式科学课程，供服务不足的纽约市中学使用。这些模块将使用学生自己制作的歌曲和动作，向他们介绍“球体音乐”。桑德森博士作为研究员将开展的工作有三个目标。首先，她将扩展一种推断银河系质量分布的新方法，该方法借用信息论中的概念，对星系中晕星的聚集进行统计分析，从而选择最适合银河系晕圈的方法。这种方法将应用于来自Gaia卫星的即将到来的数据，该卫星将精确测量银河系数百万流星的完整位置和速度。其次，她将研究小暗物质团块对潮汐流的影响，通过预测它们对流星轨道的影响，并确定哪些影响是最具诊断性的。最后，桑德森博士将完成对一种新工具的测试，该工具可以估计银河系外星系的质量，该工具基于对一种特殊类型的潮汐流的观测，这种潮汐流被称为贝壳或雨伞。在这些特殊情况下，我们可以了解流星的轨道，从而了解宿主星系的质量，即使我们只能获得关于这些遥远恒星的有限信息。来自这些新策略的信息将使桑德森博士能够将包括我们银河系在内的星系晕的结构与基于不同暗物质理论的预测进行比较。在她的工作中产生的图像、电影和计算机模拟将被纳入她将开发的中学科学课程，以帮助教育和激励新一代美国科学家。

Dr. Robyn Sanderson is awarded an NSF Astronomy and Astrophysics Postdoctoral Fellowship to carry out a program of research and education at Columbia University and New York University. Outside the Galactic disk, where most stars including our Sun reside, most of our Galaxy's mass is in its roughly spherical halo of dark matter, invisible material only known to exist through its gravitational influence. Also out there are a handful of halo stars, including the scattered remains of smaller galaxies that entered the Milky Way's dark halo only to be ripped apart by its tidal forces. The structures left behind, called tidal streams, can be used to study the distribution of the Milky Way's dark matter. Each tidal stream is made up of stars that were once neighbors in the same small galaxy, and thus now follow similar paths as they orbit the Milky Way. This common origin gives us insight into the Galaxy's mass distribution, which determines the orbits of the stream's stars: only if we guess close to the true mass distribution do the computed orbits of stars in each stream cluster together around a common progenitor. Changes to the neighboring orbits of tidal stream stars could also provide evidence for smaller-scale dark matter structure: tiny dark matter clumps too small to hold galaxies, predicted by some theories of dark matter, would deflect the paths of stream stars in predictable ways. Dr. Sanderson will develop new tools to explore the structure of dark matter halos, particularly the Milky Way's, by studying their tidal streams. These tools use state-of-the-art methods, yet are based on fundamental concepts that kids learn in middle school: why stars shine, how we make and read maps, and how gravity works. To teach these core ideas and inspire middle schoolers' interest in science, Dr. Sanderson will develop portable science lessons for use in underserved New York City middle schools. The modules will use songs and movement the students produce themselves to introduce them to the "music of the spheres."The work Dr. Sanderson will carry out as a fellow has three goals. First, she will expand a new method of inferring the Milky Way's mass distribution, which uses concepts borrowed from information theory to statistically analyze the clustering of halo stars in streams and thereby select the best fit for the Milky Way halo. This method will be applied to forthcoming data from the Gaia satellite, which will accurately measure complete positions and velocities for millions of stream stars in the Milky Way. Second, she will study the influence of small dark matter clumps on tidal streams, by predicting their effects on the orbits of the stream stars and determining which effects would be most diagnostic to observe. Finally, Dr. Sanderson will complete tests of a new tool to estimate the masses of galaxies outside our Milky Way, based on observations of a particular type of tidal stream known as a shell or umbrella. In these special cases we can understand the stream stars' orbits, and hence the mass of the host galaxy, even with only the limited information we can obtain for such distant stars. The information from these new strategies will allow Dr. Sanderson to compare the structures of galaxy halos, including our Milky Way's, to predictions based on different theories of dark matter. The images, movies, and computer simulations produced in her work will be incorporated into the middle school science lessons she will develop, to help educate and inspire a new generation of American scientists.