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