Orbital Dynamics in Asymmetric Galaxies -- Exploring the Gravitational Potential

宇宙的大部分质量不发光；我们只能通过它对发光恒星和气体的引力知道它的存在。在星系内部，这种“暗物质”对星系外部的影响最大。这个项目将研究极环星系。在这些星系中，一个由气体、恒星和尘埃组成的巨大环围绕着一个旋转的恒星内盘，就像我们的银河系一样——但是这个环垂直于中央圆盘。环和中央星系中恒星和气体的运动都受到引力的控制，包括那些由不发光物质组成的“暗晕”。如果暗晕大致是球形的，那么光环中的气体应该围绕星系中心绕圈运行，而如果暗物质位于类似银河系的圆盘中，那么光环将被引力压成椭圆形。通过观察这些运动，研究人员将确定这个光晕是大致圆形，还是形成一个扁平的圆盘。后者的发现将支持这样一种观点，即暗物质是“正常”物质（如燃烧殆尽的恒星），而不是地球上尚未观察到的弱相互作用粒子。研究人员还打算研究另一个引力问题：恒星的轨道受到的引力并不总是相同的，而是随时间而变化的。一个例子是一颗属于一个小星系的恒星，它沿着一个大得多的星系的椭圆轨道运行——我们的银河系有几个这样的卫星。当卫星俯冲到低处时，银河系增加的引力会将恒星从它的外围区域移走。PI已经开发了一种新的方法，即“不变循环”方法，将用于解决这个问题，并找出哪些恒星将留在卫星上，哪些将从卫星上剥离。然后，就有可能利用斯隆数字巡天和其他巡天收集的这些小星系周围恒星的观测数据，来测量卫星星系与银河系的引力。通过这种方法，可以估计卫星的质量，并计算出它们的“暗物质”含量。本项目由美国国家科学基金会星系外天文学和宇宙学项目（AST/EXC）资助

ABSTRACT AST 0098419SparkeMost of the Universe's mass emits no light; we know of its existence only through its gravitational pull on the luminous stars and gas. Within galaxies, this `dark matter' is most influential in the outer parts. This project will study polar ring galaxies. In these, a huge ring of gas, stars and dust encircles a rotating inner disk of stars, much like our own Milky Way -- but the ring stands perpendicular to the central disk. The motions of stars and gas in both the ring and the central galaxy are controlled by gravitational forces, including those of the `dark halo' of non-luminous matter. If the dark halo is roughly spherical, gas in the ring should orbit in circles around the galaxy center, while if the dark stuff lies in a Milky-Way-like disk, the ring will be squashed into an oval by its gravitational force. From observations these motions, the investigators will determine whether the halo is roughly round, or forms a flattened disk. The latter finding would support the idea that the dark material is 'normal' matter (such as burned-out stars), rather than weakly-interacting particles that have not yet been observed on Earth.The investigators also intend to study another gravitational problem: the orbits of stars that experience a gravitational force that is not always the same, but varies with time. An example is that of a star belonging to a small galaxy that follows an elliptical orbit around a much larger galaxy -- our Milky Way has several such satellites. As the satellite swoops in low, the increased gravitational force of the Milky Way can remove stars from its outer regions. The PI has developed a novel method, that of 'invariant loops', that will be used to tackle this problem, and to find which stars will remain with the satellite and which will be stripped from it. It will then be possible to use observations of stars around these small galaxies, now being gathered by the Sloan Digital Sky Survey and other surveys, to measure the gravitational force of the satellite galaxy in comparison to that of the Milky Way. In this way the masses of the satellites can be estimated and their 'dark matter' content calculated. Funding for this project was provided by the NSF program for Extragalactic Astronomy & Cosmology (AST/EXC).***