Impacts of Radial Mixing on the Galactic Thick and Thin Disks

TL;DR

This study uses N-body simulations to explore how radial mixing affects the metallicity gradients in the Milky Way's thick and thin disks, providing insights into their formation and evolution.

Contribution

It demonstrates how radial mixing influences metallicity gradients depending on initial disk conditions and compares model results with observational data.

Findings

Radial mixing can produce positive or negative vertical metallicity gradients.

Flaring initial disks lead to negative metallicity gradients consistent with observations.

The progenitor of the Milky Way's thick disk likely did not have a steep negative metallicity gradient.

Abstract

Using N-body simulations of the Galactic disks, we qualitatively study how the metallicity distributions of the thick and thin disk stars are modified by radial mixing induced by the bar and spiral arms. We show that radial mixing drives a positive vertical metallicity gradient in the mono-age disk population whose initial scale-height is constant and initial radial metallicity gradient is tight and negative. On the other hand, if the initial disk is flaring, with scale-height increasing with galactocentric radius, radial mixing leads to a negative vertical metallicity gradient, which is consistent with the current observed trend. We also discuss impacts of radial mixing on the metallicity distribution of the thick disk stars. By matching the metallicity distribution of N-body models to the SDSS/APOGEE data, we argue that the progenitor of the Milky Way's thick disk should not have a steep negative metallicity gradient.