Discy dwarf disruption and the shape of the Galactic halo

TL;DR

This paper investigates the formation of the Sagittarius stellar stream, revealing that apparent precession of the stream's debris plane can occur even in spherical halos due to progenitor disk wobbling, challenging assumptions about halo shape inference.

Contribution

It demonstrates that stream precession is not a definitive sign of halo asphericity and that progenitor disk dynamics significantly influence stream morphology.

Findings

Stream debris plane precession can occur in spherical halos.

Progenitor disk wobbling causes apparent stream precession.

Precession is not a reliable indicator of dark halo shape.

Abstract

The shape of the Galactic dark halo can, in principle, be inferred through modelling of stellar tidal streams in the Milky Way halo. The brightest and the longest of these, the Sagittarius stream, reaches out to large Galactocentric distances and hence can deliver the tightest constraints on the Galaxy's potential. In this contribution, we revisit the idea that the Sagittarius Stream was formed from a rotating progenitor. We demonstrate that the angle between the disk's angular momentum and the progenitor's orbital angular momentum does not remain constant throughout the disruption. Instead, it undergoes a dramatic evolution caused, in part, by the changes in the progenitor's moment of inertia tensor. We show that, even in a spherical potential, the streams produced as a result of a disky dwarf disruption appear to be "precessing". Yet, this debris plane evolution is illusory as it is solely caused by the swaying and wobbling of the progenitor's disk. Stream plane precession is therefore not an unambiguous indicator of asphericity of the dark halo.