Collaborative Research: Probing Phase-Space Structure in the Galaxy - Kapteyn's Selected Areas

最近的大面积光度测量结果表明，银河系外晕含有吸积驱动的子结构。这些已知或假定为卫星吸积残留物的结构具有长期、连贯的潮汐特征，可用于准确地模拟银河系引力势以及原始卫星的特征。然而，这样的建模研究一直受到沿潮汐特征的大角度可用运动学数据的限制。虽然径向速度计划最近才开始为为数不多的银河系潮汐尾巴解决这个问题，但还没有系统的调查开始解决横向(切向)速度(即绝对自行)，它包含了两倍的动力学信息。如果没有这些信息，动力学模型仍然缺乏约束，因此仅限于描述一系列可能的事件，而不是提供对真实事件的准确描述。在这个合作项目中，耶鲁大学的Dana Dinescu博士和弗吉尼亚大学的Steven Majewski博士将进行一次深度、高精度的绝对和相对自行测量，将在卡普廷于1906年为银河系结构研究指定的选定区域对50多条视线进行采样；事实上，卡普廷及其同事当时拍摄的照相底片将被用作第一纪元材料。目前的自行程序不能在类似的震级极限下达到这种精度，因此在探测和表征遥远的晕子结构方面受到限制。这个项目的自然运动精度、深度和天空覆盖的组合在即将到来的天体测量卫星任务GAIA和SIM之前是不会被超越的。这些任务的数据可能分别在2015年和2020年公布，而这一项目的结果将在未来3-4年公布。自然运动将得到来自光度和光谱工作以及2MASS、QUEST、SDSS和RAVE等重叠观测的径向速度、距离和金属丰度估计的补充。数据预期应用的例子包括：1)确定高度模糊的独角星座的范围和轨道运动，银河系平面上方和下方的反中心结构，2)表征人马座潮汐流的横向运动。3)探测和表征银河晕中的附加亚结构。4)确定场中众多厚盘和晕星的运动学性质作为银心距的函数。只有几个深度的、精确的(笔杆式)自行数据集是以银河系场星为中心的，而不是球状星团或矮球。该项目将大大增加铅笔波束型数据集的数量，从而在50多个不同的视线范围内系统地探测我们的银河系。使该项目成为可能的是原有的60英寸Mt.1909年拍摄的威尔逊望远镜平板与匹配的杜邦2.5米平板，从而产生了90年优秀板块尺度材料的基线。这项研究将有助于建立我们的银河系与其以前的卫星相互作用的准确模型，并有助于描述银河系的卫星系统。此外，碎片恒星的运动学数据也有可能测量暗物质晕的块状。这两个问题与冷暗物质模拟有关，该模拟目前将暗物质晕的数量相对于银河系观测卫星的数量高估了两个数量级。***

Recent results from large-area photometric surveys have shown that the Milky Way outer halo contains accretion-driven substructure. These structures of known or assumed remnants of satellite accretion, have long-lived, coherent tidal features that can be used to accurately model the Galactic gravitational potential, as well as the characteristics of the original satellite. However, such modeling studies have been limited by the meager available kinematical data over large angles along the tidal features. While radial-velocity programs have just recently begun to address this problem for the few known Galactic tidal tails, no systemic survey has begun to address the transverse (tangential) velocities (i.e., absolute proper motions), which contain twice as much dynamical information. Without this information, dynamical models remain poorly constrained, and therefore limited to describing merely a range of possible events, rather than providing an accurate description of the real event. In this collaborative project, Dr. Dana Dinescu at Yale University and Dr. Steven Majewski, at the University of Virginia, will undertake a deep, high-precision absolute and relative proper-motion survey that will sample more than 50 lines of sight in the Selected Areas designated by Kapteyn for Galactic structure studies in 1906; indeed, the photographic plates taken at that time by Kapteyn and colleagues will be used as first epoch material. Current proper-motion programs do not achieve this precision at a similar magnitude limit, and are thus limited in detecting and characterizing distant halo substructure. The combination of proper-motion precision, depth, and sky coverage of this project will not be surpassed until the upcoming astrometric satellite missions GAIA and SIM. The data from these missions may be available in 2015 and 2020 respectively, while results from this project will be available in the next 3-4 years. The proper motions will be complemented by radial velocities, distance and metallicity estimates from photometric and spectroscopic work as well as from overlapping surveys like 2MASS, QUEST, SDSS and RAVE. Examples of intended applications of the data include: 1) Determining the extent and orbital motion of the highly obscured Monoceros, anticenter structure both above and below the Galactic plane, 2) Characterizing the transverse motion of the Sagittarius tidal streams. 3) Detecting and characterizing additional substructures in the halo of the Milky Way. 4) Determining the kinematical properties of the numerous thick disk and halo stars in the fields as a function of Galacto-centric distance. There are only a few deep, precise (pencil-beam-type) proper-motion data-sets that are centered on Galactic field stars rather than on a globular cluster or a dwarf spheroidal. This project will substantially increase the number of pencil-beam-type data-sets and therefore systematically probe our Galaxy in more than 50 distinct lines-of-sight. What makes the project possible is the combination of original 60-inch Mt. Wilson telescope plates taken in 1909 with matching Du Pont 2.5-m plates, thus yielding a 90 year baseline of excellent plate scale material. This research will contribute to an accurate model of the interaction of our Galaxy with its former satellites, and help characterize the Galaxy's satellite system. Also, the kinematical data of debris stars have the potential of measuring the lumpiness of the dark matter halo. These two issues are relevant for cold dark matter simulations that currently overpredict, by two orders of magnitude, the number of dark matter halos relative to the number of observed satellites of the Milky Way. ***