Evolution over time of the Milky Way's disc shape

TL;DR

This study investigates how the Milky Way's outer disc structure, including scale length, warp, and flare, varies with stellar age, revealing insights into galaxy formation and evolution.

Contribution

It introduces a method to analyze the age dependence of Galactic disc parameters using 2MASS data and a population synthesis model, highlighting their evolution over time.

Findings

Thin disc scale length varies from 3.8 kpc to 2 kpc with age.

Warp asymmetry and node angle change with stellar age.

Outer disc exhibits significant flaring with increasing Galactocentric distance.

Abstract

Context. Galactic structure studies can be used as a path to constrain the scenario of formation and evolution of our Galaxy. The dependence with the age of stellar population parameters would be linked with the history of star formation and dynamical evolution. Aims. We aim to investigate the structures of the outer Galaxy, such as the scale length, disc truncation, warp and flare of the thin disc and study their dependence with age by using 2MASS data and a population synthesis model (the so-called Besan\c{c}on Galaxy Model). Methods. We have used a genetic algorithm to adjust the parameters on the observed colour-magnitude diagrams at longitudes 80 deg <= l <= 280 deg for |b| <= 5.5 deg. We explored parameter degeneracies and uncertainties. Results. We identify a clear dependence of the thin disc scale length, warp and flare shapes with age. The scale length is found to vary between 3.8 kpc for the youngest to about 2 kpc for the oldest. The warp shows a complex structure, clearly asymmetrical with a node angle changing with age from approximately 165 deg for old stars to 195 deg for young stars. The outer disc is also flaring with a scale height that varies by a factor of two between the solar neighbourhood and a Galactocentric distance of 12 kpc. Conclusions. We conclude that the thin disc scale length is in good agreement with the inside-out formation scenario and that the outer disc is not in dynamical equilibrium. The warp deformation with time may provide some clues to its origin.