Orbital Dynamics in Asymmetric Galaxies -- Exploring the Gravitational Potential

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

ABSTRACT AST 0098419 Sparke Most 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). ***