Deciphering the evolution of the Milky Way discs: Gaia APOGEE Kepler giant stars and the Besan\c{c}on Galaxy Model

TL;DR

This study analyzes the Milky Way's disc evolution using Gaia, APOGEE, and Kepler data, revealing complex chemo-kinematic relationships and suggesting a more intricate formation history involving mergers and migration.

Contribution

It provides a detailed chemo-kinematic characterization of Milky Way disc populations and compares observations with the Besan extbackslash c{}on Galaxy Model, highlighting its limitations.

Findings

Thin disc shows flat age-metallicity relation with increasing [$ ext{α}$/Fe] over time.

No correlation between velocities and chemical properties within populations.

Vertical velocity dispersion peaks around 8 Gyr for the thick disc.

Abstract

We investigate the properties of the double sequences of the Milky Way discs visible in the [$\alpha$/Fe] vs [Fe/H] diagram. In the framework of Galactic formation and evolution, we discuss the complex relationships between age, metallicity, [$\alpha$/Fe], and the velocity components. We study stars with measured chemical, seismic and astrometric properties from the APOGEE survey, the Kepler and Gaia satellites, respectively. We separate the [$\alpha$/Fe]-[Fe/H] diagram into 3 stellar populations: the thin disc, the high-$\alpha$ metal-poor thick disc and the high-$\alpha$ metal-rich thick disc and characterise each of these in the age-chemo-kinematics parameter space. We compare results obtained from different APOGEE data releases and using two recent age determinations. We use the Besan\c{c}on Galaxy model (BGM) to highlight selection biases and mechanisms not included in the model. The thin disc exhibits a flat age-metallicity relation while [$\alpha$/Fe] increases with stellar age. We confirm no correlation between radial and vertical velocities with [Fe/H], [$\alpha$/Fe] and age for each stellar population. Considering both samples, V$_\varphi$ decreases with age for the thin disc, while it increases with age for the h$\alpha$mp thick disc. Although the age distribution of the h$\alpha$mr thick disc is very close to that of the h$\alpha$mp thick disc between 7 and 14 Gyr, its kinematics seems to follow that of the thin disc. This feature, not predicted by the hypotheses included in the BGM, suggests a different origin and history for this population. Finally, we show that there is a maximum dispersion of the vertical velocity, $\sigma_Z$, with age for the h$\alpha$mp thick disc around 8 Gyr. The comparisons with the BGM simulations suggest a more complex chemo-dynamical scheme to explain this feature, most likely including mergers and radial migration effects