Speaker
Description
Tracing the Milky Way disc structure as a function of stellar age is key to understanding its formation history. However, fitting density models to observational data is a major challenge due to selection biases.
To avoid modelling the complex selection function, we focus on fitting stellar kinematics, using the quasi-isothermal disc distribution function. We use the APOGEE DR17 giant stars, with ages estimated from BINGO, an asteroseismology trained Bayesian Machine Learning framework (Ciucă et al. 2024, MNRAS, 528, L122), and crossmatch them with the Gaia DR3. We estimate how the scale length and radial/vertical velocity dispersions change with age by fitting a non-parametric spline function.
Our results show that the old (>~10Gyr) stellar disc has a small radial scale length (1.8 kpc) and high velocity dispersion, consistent with expected properties of the thick disc. In contrast, the young (<~8 Gyr) disc shows an increasing scale length and decreasing velocity dispersion with decreasing age, supporting an inside-out and upside-down formation scenario for the thin disc. Notably, this transition between the thin and thick discs coincides with the end of the Great Galactic Starburst, which is thought to be linked to the Gaia-Sausage-Enceladus (GSE) gas-rich merger (Ciucă et al. 2024). We also find a slight decrease in scale length at this transition epoch, consistent with the aftermath of a GSE-like gas-rich merger seen in simulations of Grand et al. (2018, MNRAS, 474, 3629). This suggests that the GSE merger may have triggered the transition from thick to thin disc formation phase.
