On the radial metallicity gradient and radial migration effect of the Galactic disk

TL;DR

This study investigates how the radial metallicity gradient varies with stellar populations and migration effects in the Galactic disk, revealing a transition at a specific [Mg/Fe] value and the influence of radial migration on metallicity distribution.

Contribution

It provides new insights into the relationship between metallicity gradients, stellar populations, and radial migration effects in the Galactic disk, especially distinguishing thin and thick disk behaviors.

Findings

Radial metallicity gradient transitions at [Mg/Fe] ~ 0.18 dex.

Positive gradients observed in high-[Mg/Fe] populations at low |Z|.

Radial migration flattens the metallicity gradient and affects high-eccentricity stars.

Abstract

We study the radial metallicity gradient $\Delta[M/H]/\Delta R_g$ as a function of [Mg/Fe] and $|Z|$ with the help of a guiding radius based on the Apache Point Observatory Galactic Evolution Experiment and Gaia and then analyze the radial migration effect on the radial metallicity gradient and metallicity-rotation gradient between the Galactic thin and thick disks. The derived trend of gradient $\Delta[M/H]/\Delta R_g$ versus [Mg/Fe] shows a transition at [Mg/Fe] $\sim 0.18$ dex, below which the gradient is negative and varies a little as [Mg/Fe] increases; however, it changes sharply in [Mg/Fe] ranges of 0.16-0.18, above which the gradient increases linearly with increasing [Mg/Fe], being a positive value at [Mg/Fe]$\gtrsim 0.22$ dex. These positive gradients in the high-[Mg/Fe] populations are found at $|Z| < 0.8$ kpc, and there are nearly no gradients toward higher $|Z|$. By comparing the metallicity distributions, the radial metallicity gradients $\Delta[M/H]/\Delta R$ and the metallicity-rotation gradients between the total sample and $|R-R_g|<2$ kpc subsample (or $|R-R_g|>2$ kpc subsample), we find that, for the thick disk, blurring flattens the gradient $\Delta[M/H]/\Delta R$ and favors metal-poor high-eccentricity stars. These stars are responsible for the measured positive metallicity-rotation gradient of the thick disk.