Quantifying radial migration in the Milky Way: Inefficient over short timescales but essential to the very outer disc beyond ~15 kpc

TL;DR

This study analyzes the extent of radial migration in the Milky Way, finding it to be limited over short timescales but significant in shaping the outer disc beyond 15 kpc, based on APOGEE data and metallicity distributions.

Contribution

It provides a quantitative assessment of radial migration's strength in the Milky Way, highlighting its inefficiency over short timescales and its role in outer disc formation.

Findings

Radial migration is limited within 4 kpc over the last 3 Gyr.

Outer disc beyond 15 kpc is consistent with migration from smaller radii.

Intrinsic metallicity gradients are approximately -0.06 dex kpc$^{-1}$.

Abstract

Stellar radial migration plays an important role in reshaping a galaxy's structure and the radial distribution of stellar population properties. In this work, we revisit reported observational evidence for radial migration and quantify its strength using the age--[Fe/H] distribution of stars across the Milky Way with APOGEE data. We find a broken age--[Fe/H] relation in the Galactic disc at $r>6$ kpc, with a more pronounced break at larger radii. To quantify the strength of radial migration, we assume stars born at each radius have a unique age and metallicity, and then decompose the metallicity distribution function (MDF) of mono-age young populations into different Gaussian components that originated from various birth radii at $r_{\rm birth}<13$ kpc. We find that, at ages of 2 and 3 Gyr, roughly half the stars were formed within 1 kpc of their present radius, and very few stars ($<5$%) were formed more than 4 kpc away from their present radius. These results suggest limited short distance radial migration and inefficient long distance migration in the Milky Way during the last 3 Gyr. In the very outer disc beyond 15~kpc, the observed age--[Fe/H] distribution is consistent with the prediction of pure radial migration from smaller radii, suggesting a migration origin of the very outer disc. We also estimate intrinsic metallicity gradients at ages of 2 and 3 Gyr of $-0.061$ dex kpc$^{-1}$ and $-0.063$ dex kpc$^{-1}$, respectively.