Impacts of a Flaring Star-forming Disc and Stellar Radial Mixing on the Vertical Metallicity Gradient

TL;DR

This study uses N-body simulations to explore how star formation history and stellar mixing influence the vertical metallicity gradient in the Milky Way's thin disc, revealing that disc flaring and radial mixing can produce observed metallicity trends.

Contribution

It demonstrates that initial disc flaring and stellar radial mixing can explain the observed vertical metallicity gradients in the Milky Way's thin disc, a novel insight into galactic chemical evolution.

Findings

Radial mixing drives a positive vertical metallicity gradient.

Disc flaring can produce a negative vertical metallicity gradient.

Predicted steeper inner disc gradient and skewed metallicity distribution for young stars.

Abstract

Using idealised N-body simulations of a Milky Way-sized disc galaxy, we qualitatively study how the metallicity distributions of the thin disc star particles are modified by the formation of the bar and spiral arm structures. The thin disc in our numerical experiments initially has a tight negative radial metallicity gradient and a constant vertical scale-height. We show that the radial mixing of stars drives a positive vertical metallicity gradient in the thin disc. On the other hand, if the initial thin disc is flared, with vertical scale-height increasing with galactocentric radius, the metal poor stars originally in the outer disc become dominant in regions above the disc plane at every radii. This process can drive a negative vertical metallicity gradient, which is consistent with the current observed trend. This model mimics a scenario where the star-forming thin disc was flared in the outer region at earlier epochs. Our numerical experiment with an initial flared disc predicts that the negative vertical metallicity gradient of the mono-age relatively young thin disc population should be steeper in the inner disc, and the radial metallicity gradient of the mono-age population should be shallower at greater heights above the disc plane. We also predict that the metallicity distribution function of mono-age young thin disc populations above the disc plane would be more positively skewed in the inner disc compared to the outer disc.