Imprints of radial migration on the Milky Way's metallicity distribution functions

TL;DR

This study demonstrates that radial migration of stars significantly influences the Milky Way's metallicity distribution functions, explaining observed skewness changes and invariant high-[alpha/Fe] populations through simulation and data analysis.

Contribution

It provides a novel simulation-based explanation for the impact of radial migration on the Milky Way's metallicity distribution and alpha-element abundance patterns.

Findings

Radial migration causes the MDF shape to change with radius.

Migrated stars form a large fraction of the outer disk's stars.

Migration explains the invariant high-[alpha/Fe] MDFs across the galaxy.

Abstract

Recent analysis of the SDSS-III/APOGEE Data Release 12 stellar catalogue has revealed that the Milky Way's metallicity distribution function (MDF) changes shape as a function of radius, transitioning from being negatively skewed at small Galactocentric radii to positively skewed at large Galactocentric radii. Using a high resolution, N-body+SPH simulation, we show that the changing skewness arises from radial migration - metal-rich stars form in the inner disk and subsequently migrate to the metal-poorer outer disk. These migrated stars represent a large fraction (> 50%) of the stars in the outer disk; they populate the high metallicity tail of the MDFs and are, in general, more metal-rich than the surrounding outer disk gas. The simulation also reproduces another surprising APOGEE result: the spatially invariant high-[alpha/Fe] MDFs. This arises in the simulation from the migration of a population formed within a narrow range of radii (3.2+/-1.2 kpc) and time (8.8+/-0.6 Gyr ago), rather than from spatially extended star formation in a homogeneous medium at early times. These results point toward the crucial role radial migration has played in shaping our Milky Way.