Classifying orbits in a new dynamical model describing motion in a prolate or an oblate elliptical galaxy

Abstract

The regular or chaotic character of orbits of stars moving in the meridional plane (R,z) of an axially symmetric elliptical galaxy with a dense, massive spherical nucleus and a dark matter halo component is under investigation. In particular, we explore how the flattening of an elliptical galaxy influences the overall orbital structure of the system, by computing in each case the percentage of chaotic orbits, as well as the percentages of orbits composing the main regular families. In an attempt to discriminate safely and with certainty between regular and chaotic motion, we use the Smaller ALingment Index (SALI) method to extensive samples of orbits obtained by integrating numerically the basic equations of motion as well as the variational equations. In addition, a technique which is based mainly on the field of spectral dynamics that utilizes the Fourier transform of the time series of each coordinate is used for classifying the regular orbits into different families and also to recognize the secondary resonances that usually bifurcate from them. Three cases are considered in our work: (i) the case where the elliptical galaxy is prolate (ii) the case where a spherically symmetric elliptical galaxy is present and (iii) the case where the elliptical galaxy has an oblate shape. Comparison between the current results and early related work is also made.