The Flattening Metallicity Gradient in the Milky Way's Thin Disk

TL;DR

This study analyzes the metallicity gradients in the Milky Way's thin disk using a large stellar sample, revealing evidence of radial migration and differences between thin and thick disk populations.

Contribution

It provides the first large-scale analysis of metallicity gradients as a function of age and orbital parameters, highlighting the role of radial migration in the Milky Way's disk evolution.

Findings

Gradients become shallower for older thin disk stars.

Radial migration (churning) influences metallicity distribution.

Thick disk shows nearly flat or slightly positive gradients.

Abstract

We calculate the ages, orbits and phase-space coordinates for a sample of $\sim$4 million LAMOST and Gaia stars. The ages are crossmatched and compared with values from two other popular age catalogs which derive the ages using different methods. Of these $\sim$4 million stars, we select a sample of 1.3 million stars and investigate their radial metallicity gradients (as determined by orbital radii) as a function of their ages. This analysis is performed on various subsets of the data split by chemistry and orbital parameters. We find that commonly used selections for ``thin disk'' stars (such as low-$\alpha$ chemistry or vertically thin orbits) yield radial metallicity gradients which generally grow shallower for the oldest stars. We interpret this as a hallmark feature of radial migration (churning). Constraining our sample to very small orbital Z$_{max}$ (the maximal height of a star's integrated orbit) makes this trend most pronounced. A chemistry-based ``thin disk'' selection of $\alpha$-poor stars displays the same trend, but to a lesser extent. Intruigingly, we find that ``thick disk'' selections in chemistry and Z$_{max}$ reveal a slightly positive radial metallicity gradients which seem similar in magnitude at all ages. This may imply that the thick disk population is well mixed in age, but not in radius. This finding could help constrain conditions during the early epochs of Milky Way formation, and shed light on processes such as the accretion and reaccretion of gasses of different metallicities.