[Paper Review] Revisiting a disky origin for the faint branch of the Sagittarius stellar stream

Pierre-Antoine Oria, Rodrigo Ibata+3 more|arXiv (Cornell University)|May 30, 2022

Stellar, planetary, and galactic studies27 references13 citations

TL;DR

This paper proposes that the faint branch of the Sagittarius stellar stream originated from a high-angular-momentum, disky spiral structure in the progenitor dwarf galaxy 3 Gyr ago, oriented nearly perpendicular to both the Milky Way disk and the Sgr orbital plane. N-body simulations with test particles show that such a configuration naturally produces the observed bifurcation, with the faint branch populated by stars stripped from this tilted disk, offering a self-consistent origin for the stream's double-armed morphology without requiring current rotation in the remnant.

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.

Motivation & Objective

• To explain the origin of the faint branch in the bifurcated Sagittarius stellar stream, a long-standing puzzle in galactic dynamics.
• To test whether a disk-like component in the Sgr progenitor could naturally produce the observed faint branch morphology.
• To explore if a tilted, spiral-disrupted disk could account for the kinematic and spatial features of the faint stream without requiring current rotation in the Sgr remnant.
• To provide a physically plausible, self-consistent model that can be combined with existing Sgr stream simulations to reproduce both bright and faint branches.

Proposed method

• Running N-body simulations of the Sgr dwarf falling into the Milky Way and LMC potential over the last 3 Gyr, using the V21 model as a reference.
• Adding test particles with diverse initial positions, energies, and angular momenta to identify those ending up in the faint branch.
• Generating synthetic stellar disks using Agama with specific scale radius (0.9 kpc), scale height (0.18 kpc), and velocity dispersion (4 km/s), then rotating them at various inclinations relative to the Sgr orbital plane.
• Applying coordinate transformations using rotation matrices around the x- and y-axes to simulate disks misaligned with the orbital plane.
• Selecting particles matching the faint branch using polynomial fits to Gaia EDR3 data (Ramos et al. 2021) in the (˜Λ⊙,˜β⊙) coordinate system.
• Running a follow-up simulation with self-gravity by replacing some stellar particles with massive particles to test the stability and evolution of the disky component.

Experimental results

Research questions

• RQ1 Can a tilted, spiral-like disk component in the Sgr progenitor naturally produce the faint branch of the bifurcated stream?
• RQ2 What initial conditions (position, energy, angular momentum) lead to particles populating the faint branch in N-body simulations?
• RQ3 Does a disky origin for the faint branch alleviate the need for current rotation in the Sgr remnant, which contradicts observations?
• RQ4 Can a self-consistent model with a massive, rotating disk component reproduce both the faint branch and the observed stream kinematics?

Key findings

• Particles in the faint branch predominantly originate from a single, high-angular-momentum disk component in the Sgr progenitor, oriented nearly perpendicular to both the Sgr orbital plane and the Milky Way disk plane.
• The initial configuration of these particles corresponds to a spiral structure already present 3 Gyr ago, which could result from tidal perturbations or internal dynamics within the progenitor.
• The model successfully reproduces the observed morphology of the faint branch, including its parallel, non-X-shaped geometry, avoiding the pitfalls of orbital precession or inner rotation models.
• A self-gravity simulation confirms that the disky component remains stable and continues to produce the faint branch, supporting its viability as a physical origin.
• The faint branch can be populated independently from the bright branch, enabling its pairing with a separate, more massive spherical component to reproduce the full stream.
• The model is consistent with observed metallicity similarity between the faint and bright branches, as both could originate from the same stellar population.