The Orbital Evolution Induced by Baryonic Condensation in Triaxial Halos

Using spectral methods, we analyse the orbital structure of dark matter (DM) in N -body simulations in an effort to understand the physical processes that drive the evolution of dark matter halo shapes caused by growing central masses. A longstanding issue is whether the change in the shapes of DM halos is the result of chaotic scattering of the major family of box orbits that serves as the back-bone of a triaxial system, or whether they change shape in response to the evolving galactic potential. We use the characteristic orbital frequencies to classify orbits into major orbital families, to quantify orbital shapes, and to identify resonant orbits and chaotic orbits. We show that regardless of the distribution of the baryonic component, the shape of a DM halo changes primarily due to changes in the shapes of individual orbits within a given family. Orbits with small pericentric radii are more likely to change both their orbital type and shape than orbits with large pericentric radii. Whether the evolution is regular (and reversible) or chaotic (and irreversible), depends primarily on the radial distribution of the baryonic component. A massive, compact central mass results in chaotic scattering of a large enough fraction of both box and long-axis tube orbits, even at fairly large pericentric distances, such that the evolution is not reversible. Frequency maps show that the growth of a disk causes a significant fraction of halo particles to become associated with major global orbital resonances.