Stray, swing and scatter: angular momentum evolution of orbits and streams in aspherical potentials

TL;DR

This paper investigates how the angular momentum and orbital planes of stellar streams evolve in aspherical gravitational potentials, providing analytic models to connect stream behavior with the shape of dark matter halos, aiding in Galactic structure inference.

Contribution

It introduces analytic expressions linking stream pole evolution to potential flattening, applicable to both near-circular and eccentric orbits, and validates these models with N-body simulations.

Findings

Stream poles exhibit straying and swinging behavior in aspherical potentials.

Stream broadening correlates with differential orbital precession and potential flattening.

Models agree well with N-body simulation results and can inform Galactic potential constraints.

Abstract

In aspherical potentials orbital planes continuously evolve. The gravitational torques impel the angular momentum vector to precess, that is to slowly stray around the symmetry axis, and nutate, i.e. swing up and down periodically in the perpendicular direction. This familiar orbital pole motion - if detected and measured - can reveal the shape of the underlying gravitational potential, the quantity only crudely gauged in the Galaxy so far. Here we demonstrate that the debris poles of stellar tidal streams show a very similar straying and swinging behavior, and give analytic expressions to link the amplitude and the frequency of the pole evolution to the flattening of the dark matter distribution. While these results are derived for near-circular orbits, we show they are also valid for eccentric orbits. Most importantly, we explain how the differential orbital plane precession leads to the broadening of the stream and show that streams on polar orbits ought to scatter faster. We provide expressions for the stream width evolution as a function of the axisymmetric potential flattening and the angle from the symmetry plane and prove that our models are in good agreement with streams produced in N-body simulations. Interestingly, the same intuition applies to streams whose progenitors are on short or long-axis loops in a triaxial potential. Finally, we present a compilation of the Galactic cold stream data, and discuss how the simple picture developed here, along with stream modelling, can be used to constrain the symmetry axes and flattening of the Milky Way.