On the Radial Metallicity Gradient and Radial Migration Effect of the Galactic Disk

Abstract

We study the radial metallicity gradient Δ[M/H]/ΔR _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 Δ[M/H]/ΔR _g versus [Mg/Fe] shows a transition at [Mg/Fe] ∼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] ≳ 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 Δ[M/H]/ΔR, 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 Δ[M/H]/ΔR and favors metal-poor high-eccentricity stars. These stars are responsible for the measured positive metallicity-rotation gradient of the thick disk.