Spectroscopic age estimates for APOGEE red-giant stars: Precise spatial and kinematic trends with age in the Galactic disc

Over the last few years, many studies have found an empirical relationship between the abundance of a star and its age. Here we estimate spectroscopic stellar ages for 178 825 red-giant stars observed by the APOGEE survey with a median statistical uncertainty of 17%. To this end, we use the supervised machine learning technique XGBoost, trained on a high-quality dataset of 3060 red-giant and red-clump stars with asteroseismic ages observed by both APOGEE and Kepler. After verifying the obtained age estimates with independent catalogues, we investigate some of the classical chemical, positional, and kinematic relationships of the stars as a function of their age. We find a very clear imprint of the outer-disc flare in the age maps and confirm the recently found split in the local age-metallicity relation. We present new and precise measurements of the Galactic radial metallicity gradient in small age bins between 0.5 and 12 Gyr, confirming a steeper metallicity gradient for $\sim2-5$ Gyr old populations and a subsequent flattening for older populations mostly produced by radial migration. In addition, we analyse the dispersion about the abundance gradient as a function of age. We find a clear power-law trend (with an exponent $β\approx0.15$) for this relation, indicating a relatively smooth radial migration history in the Galactic disc over the past $7-9$ Gyr. Departures from this power law may possibly be related to the Gaia Enceladus merger and passages of the Sagittarius dSph galaxy. Finally, we confirm previous measurements showing a steepening in the age-velocity dispersion relation at around $\sim9$ Gyr, but now extending it over a large extent of the Galactic disc (5 kpc $<R_{\rm Gal}<13$ kpc). [Abridged]