Vertical disc heating in Milky Way-sized galaxies in a cosmological context

TL;DR

This study uses high-resolution cosmological simulations to investigate the main mechanisms behind vertical disc heating in Milky Way-sized galaxies, finding bars as the dominant internal driver and external satellites as significant external contributors.

Contribution

It provides a comprehensive analysis of vertical disc heating mechanisms in a cosmological context, highlighting the dominant role of bars and external satellites based on new high-resolution simulations.

Findings

Bars are the primary internal heating mechanism in most cases.

External satellite perturbations significantly contribute to disc heating.

Newborn stars tend to have cooler orbits over time, affecting the age-velocity dispersion relation.

Abstract

Vertically extended, high velocity dispersion stellar distributions appear to be a ubiquitous feature of disc galaxies, and both internal and external mechanisms have been proposed to be the major driver of their formation. However, it is unclear to what extent each mechanism can generate such a distribution, which is likely to depend on the assembly history of the galaxy. To this end, we perform 16 high resolution cosmological-zoom simulations of Milky Way-sized galaxies using the state-of-the-art cosmological magneto-hydrodynamical code \textlcsc{AREPO}, and analyse the evolution of the vertical kinematics of the stellar disc in connection with various heating mechanisms. We find that the bar is the dominant heating mechanism in most cases, whereas spiral arms, radial migration, and adiabatic heating from mid-plane density growth are all sub-dominant. The strongest source, though less prevalent than bars, originates from external perturbations from satellites/sub-halos of masses log$_{10} (M/\rm M_{\odot}) \gtrsim 10$. However, in many simulations the orbits of newborn star particles become cooler with time, such that they dominate the shape of the age-velocity dispersion relation and overall vertical disc structure unless a strong external perturbation takes place.