Numerical Simulation of a possible origin of the positive radial metallicity gradient of the thick disk

TL;DR

This study uses cosmological simulations to explore how age, metallicity, and stellar motions influence the positive radial metallicity gradient observed in the thick disk at large heights.

Contribution

It demonstrates that the positive metallicity gradient in the thick disk can be explained by stellar age, velocity dispersion, and disk flaring effects in a cosmological context.

Findings

Positive metallicity gradient at high |z| due to younger, metal-rich stars reaching outer regions

Flaring of the disk causes more young, metal-rich stars to be at larger radii at high |z|

Simulation results align with observed gradients in the Milky Way thick disk

Abstract

We analyze the radial and vertical metallicity and [alpha/Fe] gradients of the disk stars of a disk galaxy simulated in a fully cosmological setting with the chemodynamical galaxy evolution code, GCD+. We study how the radial abundance gradients vary as a function of height above the plane and find that the metallicity ([alpha/Fe]) gradient becomes more positive (negative) with increasing height, changing sign around 1.5 kpc above the plane. At the largest vertical height (2 < |z| < 3 kpc), our simulated galaxy shows a positive radial metallicity gradient. We find that the positive metallicity gradient is caused by the age-metallicity and age-velocity dispersion relation, where the younger stars have higher metallicity and lower velocity dispersion. Due to the age-velocity dispersion relation, a greater fraction of younger stars reach |z| > 2 kpc at the outer region, because of the lower gravitational restoring force of the disk, i.e. flaring. As a result, the fraction of younger stars with higher metallicity due to the age-metallicity relation becomes higher at the outer radii, which makes the median metallicity higher at the outer radii. Combining this result with the recently observed age-metallicity and age-velocity dispersion relation for the Milky Way thick disk stars suggested by Haywood et al. (2013), we argue that the observed (small) positive radial metallicity gradient at large heights of the Milky Way disk stars can be explained by the flaring of the younger thick and/or thin disk stars.