Origin and structure of the Galactic disc(s)

TL;DR

This study uses a chemical and dynamical model of the Milky Way to explore the origin and structure of the Galactic disc(s), suggesting a non-violent formation history consistent with observed properties.

Contribution

It demonstrates that the chemical properties of the Galactic disc can be explained without invoking violent events, challenging previous theories about the thick disc's origin.

Findings

The model reproduces the properties of the thick disc.

The chemical composition does not indicate a violent formation process.

The thin and thick discs can be explained by internal evolution.

Abstract

We examine the chemical and dynamical structure in the solar neighbourhood of a model Galaxy that is the endpoint of a simulation of the chemical evolution of the Milky Way in the presence of radial mixing of stars and gas. Although the simulation's star-formation rate declines monotonically from its unique peak and no merger or tidal event ever takes place, the model replicates all known properties of a thick disc, as well as matching special features of the local stellar population such as a metal-poor extension of the thin disc that has high rotational velocity. We divide the disc by chemistry and relate this dissection to observationally more convenient kinematic selection criteria. We conclude that the observed chemistry of the Galactic disc does not provide convincing evidence for a violent origin of the thick disc, as has been widely claimed.