Exploring the pertubed Milky Way disk and the substructures of the outer disk

TL;DR

This study reveals that the Milky Way's disk exhibits extended spiral features in phase space caused by past dwarf galaxy interactions, affecting star motions and disk structure across large galactic radii.

Contribution

It demonstrates the widespread presence of phase spiral features beyond the solar neighborhood and links these to dwarf galaxy encounters through simulations.

Findings

Spiral features extend up to 15 kpc from the Galactic center.

The phase spiral explains observed asymmetric velocity substructures.

Simulations show dwarf galaxy impacts induce disk perturbations.

Abstract

The recent discovery of a spiral feature in $Z-V_Z$ phase plane in the solar neighborhood implies that the Galactic disk has been remarkably affected by a dwarf galaxy passing through it some hundreds of millions of years ago. Using 429,500 LAMOST K giants stars, we show that the spiral feature exits not only in the solar vicinity; it also extends to about 15 kpc from the Galactic center, and then disappears beyond this radius. Moreover, we find that when the spiral features in a plot of $V_\phi$ as a function of position in the $Z-V_Z$ plane, at various Galactocentric radii, are re-mapped to $R-Z$ plane, the spiral can explain well the observed asymmetric velocity substructures. This is evidence that the phase spiral features are the same as the bulk motions found in previous as well as this work. Test-particle simulations and N-body simulations show that an encounter with a dwarf galaxy a few hundred million years ago will induce a perturbation in the Galactic disk. In addition, we find that the last impact of Sgr dSph can also contribute to the flare. As a consequence of the encounter, the distribution function of disk stars at a large range of radii is imprinted by the gravitational perturbation.