Transient spiral structure and the disc velocity substructure in Gaia DR2

TL;DR

This paper demonstrates that transient winding spiral structures in the Milky Way can naturally produce the observed velocity substructures, including ridges and Hercules-like features, as revealed by Gaia DR2 data.

Contribution

The study introduces a test particle model of winding transient spiral structures that successfully reproduces observed velocity substructures in Gaia DR2 data.

Findings

Winding spiral models reproduce observed velocity ridges.

Transient spirals can generate Hercules-like kinematic features.

Model matches velocity distributions in solar neighborhood and along the Galactic anti-center.

Abstract

The second data release from ESA's Gaia mission has revealed many ridge-like structures in the velocity distribution of the Milky Way. We show that these can arise naturally from winding transient spiral structure that is commonly seen in N-body simulations of disk galaxies. We construct test particle models of the winding spiral structure, and compare the resulting distribution of orbits with the observed two-dimensional velocity distribution in the extended solar neighbourhood and with the distribution of rotational velocities over 8 kpc along the Sun--Galactic-centre--Galactic anti-centre line. We show that the ridges in these observations are well reproduced by the winding spiral model. Additionally, we demonstrate that the transient winding spiral potential can create a Hercules-like feature in the kinematics of the solar neighbourhood, either alone, or in combination with a long-slow bar potential.