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

Abstract

ABSTRACT We use the Gaia eDR3 data and legacy spectroscopic surveys to map the Milky Way disc substructure towards the Galactic Anticentre at heliocentric distances $d\ge 10\, \rm {kpc}$. We report the discovery of multiple previously undetected new filaments embedded in the outer disc in highly extincted regions. Stars in these overdensities have distance gradients expected for disc material and move on disc-like orbits with $v_{\phi }\sim 170{-}230\, \rm {km\, s^{-1}}$, 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 (disc streams, ‘feathers’). Due to the long time-scale 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 structures should help distinguish between the two possible morphologies.