# The structural evolution of galaxies with both thin and thick discs

## TL;DR

This study uses N-body simulations to explore how the growth of thin and thick galactic discs influences galaxy structure, bar formation, and age distribution, providing insights into Milky Way-like galaxy evolution.

## Contribution

It introduces controlled simulation models of disc galaxy evolution with thick and thin components, revealing their impact on galaxy morphology and dynamics.

## Key findings

- Double-exponential vertical profiles similar to the Milky Way.
- Thick discs delay and influence bar formation.
- Vertical age distribution shaped by bar buckling.

## Abstract

We perform controlled N-body simulations of disc galaxies growing within live dark matter (DM) haloes to present-day galaxies that contain both thin and thick discs. We consider two types of models: a) thick-disc initial conditions to which stars on near-circular orbits are continuously added over ~10 Gyr and b) models in which the birth velocity dispersion of stars decreases continuously over the same time-scale. We show that both schemes produce double-exponential vertical profiles similar to that of the Milky Way (MW). We indicate how the spatial age structure of galaxies can be used to discriminate between scenarios. We show that the presence of a thick disc significantly alters and delays bar formation and thus makes possible models with a realistic bar and a high baryon-to-DM mass ratio in the central regions, as required by microlensing constraints. We examine how the radial mass distribution in stars and DM is affected by disc growth and non-axisymmetries. We discuss how bar buckling shapes the vertical age distribution of thin- and thick-disc stars in the bar region. The extent to which the combination of observationally motivated inside-out growth histories and cosmologically motivated dark halo properties leads to the spontaneous formation of non-axisymmetries that steer the models towards present-day MW-like galaxies is noteworthy.