Kinematics beats dust: unveiling nested substructure in the perturbed outer disc of the Milky Way

TL;DR

This study uses Gaia eDR3 data to discover new filamentary substructures in the Milky Way's outer disc, revealing complex dynamics likely caused by satellite impacts and challenging the idea of a quiescent thin disc.

Contribution

It uncovers previously undetected filaments in the outer Milky Way disc and interprets their origins as satellite-induced perturbations, providing new insights into Galactic disc dynamics.

Findings

Discovered multiple new filaments in the outer disc.

Stars in these structures have disc-like orbits with small energy spreads.

Structures may be long-lived, phase-mixed features or projection effects.

Abstract

We use $Gaia$ eDR3 data and legacy spectroscopic surveys to map the Milky Way disc substructure towards the Galactic Anticenter at heliocentric distances $d\geq10\,\rm{kpc}$. We report the discovery of multiple previously undetected new filaments embedded in the outer disc in highly extincted regions. Stars in these over-densities have distance gradients expected for disc material and move on disc-like orbits with $v_{\phi}\sim170-230\,\rm{km\,s^{-1}}$, showing small spreads in energy. Such a morphology argues against a quiescently growing Galactic thin disc. Some of these structures are interpreted as excited outer disc material, kicked up by satellite impacts and currently undergoing phase-mixing ("feathers"). Due to the long timescale in the outer disc regions, these structures can stay coherent in configuration space over several Gyrs. We nevertheless note that some of these structures could also be folds in the perturbed disc seen in projection from the Sun's location. A full 6D phase-space characterization and age dating of these structure should help distinguish between the two possible morphologies.