Stream Fanning and Bifurcations: Observable Signatures of Resonances in Stellar Stream Morphology

TL;DR

This paper investigates how resonant orbit families in static, non-spherical gravitational potentials can produce observable stream features like fans and bifurcations, offering a new way to probe the Milky Way's dark matter halo shape.

Contribution

It introduces a novel numerical method to measure libration frequencies and links specific stream morphologies to resonant orbit structures in the Galactic potential.

Findings

Resonant orbit families can produce stream fans and bifurcations.

These features depend on the dark matter halo potential shape.

Potential to constrain the Milky Way's gravitational potential shape.

Abstract

Recent observations have revealed a trove of unexpected morphological features in many of the Milky Way's stellar streams. Explanations for such features include time-dependent deformations of the Galactic gravitational potential, local disruptions induced by dark matter substructure, and special configurations of the streams' progenitors. In this paper, we study how these morphologies can also arise in certain static, non-spherical gravitational potentials that host a subset of resonantly-trapped orbit families. The transitions, or separatrices, between these orbit families mark abrupt discontinuities in the orbital structure of the potential. We develop a novel numerical approach for measuring the libration frequencies of resonant and near-resonant orbits, and apply it to study the evolution of stellar streams on these orbits. We reveal two distinct morphological features that arise in streams on near-resonant orbits: fans, that come about due to a large spread in the libration frequencies near a separatrix; and bifurcations, that arise when a separatrix splits the orbital distribution of the stellar stream between two (or more) distinct orbit families. We demonstrate that these effects can arise in some Milky Way streams for certain choices of the dark matter halo potential, and discuss how this might be used to probe and constrain the global shape of the Milky Way's gravitational potential.