An Asymptotic Matching Method for Analyzing Non-Planar Orbits in Disk Galaxies

Abstract

Modeling the orbital dynamics of objects in galactic disks is crucial to understanding the stability and evolution of disk galaxies. While studies of galactic orbits are largely dominated by $N$-body simulations, perturbative analytical models offer a computationally inexpensive and conceptually insightful way of analyzing galactic dynamics. We utilize perturbation theory and the method of matched asymptotics to develop a technique by which the vertical motion of a point mass perpendicular to a thin axisymmetric disk galaxy can be computed analytically to high precision. The objective of this study is to provide an accurate model of non-planar dynamics that could be employed in galactic simulations to bypass computationally expensive integration. We construct a general solution for the dynamics at small $z$ displacements from the plane of the disk using perturbation theory. We solve for the dynamics at very large displacements from the plane of the disk and construct a function that interpolates the solutions at each length scale. To demonstrate the method's utility, we apply it to two commonly used models for the vertical structure of a galactic disk, the exponential model and the isothermal model. Finally, using the principle of adiabatic invariance, we analyze how to relate variations in radial motion to variations in the vertical motion and discuss possible applications of the model in numerical astrophysics.