Separatrix Divergence of Stellar Streams in Galactic Potentials

TL;DR

This paper studies how stellar streams near orbit boundaries in flattened galactic potentials diffuse faster due to separatrix effects, offering a new method to map galaxy potentials using stream morphology.

Contribution

It demonstrates that separatrices cause increased diffusion of stellar streams and proposes a novel approach to map galactic potentials through stream analysis.

Findings

Streams near separatrices diffuse more rapidly.

At least one previous stream-fanning observation is caused by separatrix divergence.

The study provides a foundation for using stream morphology to constrain galactic potential shapes.

Abstract

Flattened axisymmetric galactic potentials are known to host minor orbit families surrounding orbits with commensurable frequencies. The behavior of orbits that belong to these orbit families is fundamentally different than that of typical orbits with non-commensurable frequencies. We investigate the evolution of stellar streams on orbits near the boundaries between orbit families (separatrices) in a flattened axisymmetric potential. We demonstrate that the separatrix divides these streams into two groups of stars that belong to two different orbit families, and that as a result, these streams diffuse more rapidly than streams that evolve elsewhere in the potential. We utilize Hamiltonian perturbation theory to estimate both the timescale of this effect and the likelihood of a stream evolving close enough to a separatrix to be affected by it. We analyze two prior reports of stream-fanning in simulations with triaxial potentials, and conclude that at least one of them is caused by separatrix divergence. These results lay the foundation for a method of mapping the orbit families of galactic potentials using the morphology of stellar streams. Comparing these predictions with the currently known distribution of streams in the Milky Way presents a new way of constraining the shape of our Galaxy's potential and distribution of dark matter.