Quantifying radial migration in the Milky Way: inefficient over short time-scales but essential to the very outer disc beyond 15 kpc

Lian, Jianhui; Zasowski, Gail; Hasselquist, Sten; Holtzman, Jon A.; Boardman, Nicholas; Cunha, Katia; Fernández-Trincado, José G.; Frinchaboy, Peter M.; Garcia-Hernandez, D. A.; Nitschelm, Christian; Lane, Richard R.; Thomas, Daniel; Zhang, Kai
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Monthly Notices of the Royal Astronomical Society

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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 rbirth < 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 per cent) 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 and -0.063 dex kpc-1, respectively.
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