Revisiting a disky origin for the faint branch of the Sagittarius stellar stream

TL;DR

This study uses N-body simulations to explore the origin of the faint bifurcated branch of the Sagittarius stellar stream, proposing a disky progenitor with spiral features as a plausible source.

Contribution

It introduces a novel simulation approach showing that a disky component in Sgr can produce the faint stream bifurcation observed today.

Findings

High angular momentum particles form the faint branch.

Faint branch particles originate from a single disk plane.

Spiral features in the progenitor predate the stream bifurcation.

Abstract

We investigate ways to produce the bifurcation observed in the stellar stream of the Sagittarius dwarf galaxy (Sgr). Our method consists in running $N$-body simulations of Sgr falling into the Milky Way for the last 3~Gyr, with added test particles on disk orbits that span a wide range of initial positions, energies, and angular momenta. We find that particles that end up in the faint branch are predominantly high angular momentum particles that can all originate from a single plane within the progenitor, nearly perpendicular both to the orbital plane of the progenitor and to the Milky Way stellar disk. Their original configuration at the start of the simulation corresponds to spiral features already present 3~Gyr ago, which could be, e.g., the result of a disk-like component being tidally perturbed, or the tidal tails of a satellite being disrupted within Sgr. We then run a simulation including the self-gravity of this disky component. Despite the remaining ambiguity of its origin, this disk component of the Sgr dwarf with spiral over-densities provides a first step towards a working model to reproduce the observed faint branch of the bifurcated Sgr stream.