Modeling disc non-axisymmetries: multiple patterns, radial migration, and thick discs

TL;DR

This study uses simulations to explore how non-axisymmetric features like spirals and bars influence galactic disc dynamics, radial migration, and thick disc formation, finding migration alone insufficient for thick disc creation.

Contribution

It demonstrates that multiple patterns and radial migration do not produce enough thickening, challenging previous theories on thick disc formation through stellar migration.

Findings

Multiple patterns develop in all models.

Radial migration causes significant stellar redistribution.

Discs do not thicken enough to match observed thick discs.

Abstract

Disc non-axisymmetrc components, such as spirals and central bars, are nowadays known to play an important role in shaping galactic discs. Here we use Tree-SPH N-body simulations to examine the effect of these perturbers on two aspects: the occurrence of multiple patterns in discs and the effects of radial migration on disc thickening. We find that, in addition to a central bar, multiple spiral patterns and lopsided modes develop in all models. Interaction among these asymmetric features results in a large scale stellar migration. However, we show that, despite the strong radial mixing, discs cannot be thickened sufficiently to match observed thick discs. We relate this to the adiabatic cooling as stars migrate radially outwards. We also find that the bulge contribution to a thick-disc component for an Sa-type galaxy at ~2.5 disc scale-lengths is less than 1% and zero in the case of a Milky Way-like, Sb-type. Our findings cast doubt on the plausibility of thick disc formation via stellar radial migration.