Thermal-Instability-Driven Turbulent Mixing in Galactic Disks: I. Effective Mixing of Metals

TL;DR

This study uses magnetohydrodynamical simulations to demonstrate that thermal instability-driven turbulence efficiently mixes metals in galactic disks, significantly impacting their chemical evolution.

Contribution

It provides the first quantitative analysis showing thermal instability-driven turbulence as an effective mechanism for metal mixing in galactic disks.

Findings

Thermal instability-driven turbulence rapidly homogenizes metals.

Mixing timescales are comparable to local orbital periods.

Magnetic fields and spiral potentials do not significantly hinder mixing.

Abstract

Observations show that radial metallicity gradients in disk galaxies are relatively shallow, if not flat, especially at large galactocentric distances and for galaxies in the high-redshift universe. Given that star formation and metal production are centrally concentrated, this requires a mechanism to redistribute metals. However, the nature of this mechanism is poorly understood, let alone quantified. To address this problem, we conduct magnetohydrodynamical simulations of a local shearing sheet of a thin, thermally unstable, gaseous disk driven by a background stellar spiral potential, including metals modeled as passive scalar fields. Contrary to what a simple \alpha\ prescription for the gas disk would suggest, we find that turbulence driven by thermal instability is very efficient at mixing metals, regardless of the presence or absence of stellar spiral potentials or magnetic fields. The timescale for homogenizing randomly distributed metals is comparable to or less than the local orbital time in the disk. This implies that turbulent mixing of metals is a significant process in the history of chemical evolution of disk galaxies.