The Launching of Cold Clouds by Galaxy Outflows II: The Role of Thermal Conduction

TL;DR

This study investigates how electron thermal conduction influences the evolution of cold clouds in galaxy outflows, revealing that conduction can both cause evaporation and extend cloud lifetimes, affecting their acceleration and survival.

Contribution

The paper provides a detailed parameter study of thermal conduction effects on cold clouds in galactic outflows, offering fits for cloud lifetimes and velocities for use in large-scale simulations.

Findings

Thermal conduction causes cold cloud evaporation and can extend their lifetimes.

Cloud acceleration is limited due to compression from evaporation.

High-column-density clouds survive longer than low-column-density ones.

Abstract

We explore the impact of electron thermal conduction on the evolution of radiatively-cooled cold clouds embedded in flows of hot and fast material, as occur in outflowing galaxies. Performing a parameter study of three-dimensional adaptive mesh refinement hydrodynamical simulations, we show that electron thermal conduction causes cold clouds to evaporate, but it can also extend their lifetimes by compressing them into dense filaments. We distinguish between low column-density clouds, which are disrupted on very short times, and high-column density clouds with much-longer disruption times that are set by a balance between impinging thermal energy and evaporation. We provide fits to the cloud lifetimes and velocities that can be used in galaxy-scale simulations of outflows, in which the evolution of individual clouds cannot be modeled with the required resolution. Moreover, we show that the clouds are only accelerated to a small fraction of the ambient velocity because compression by evaporation causes the clouds to present a small cross-section to the ambient flow. This means that either magnetic fields must suppress thermal conduction, or that the cold clouds observed in galaxy outflows are not formed of cold material carried out from the galaxy.