The Launching of Cold Clouds by Galaxy Outflows. IV. Cosmic-Ray-Driven Acceleration

TL;DR

This study uses simulations to show that cosmic rays can efficiently accelerate cold clouds in galactic outflows, with slow cloud destruction and minimal impact from radiative cooling.

Contribution

It demonstrates that cosmic rays can serve as the main driver for accelerating cold clouds in galaxy outflows, a novel insight into outflow dynamics.

Findings

Cosmic-ray pressure builds up at cloud boundaries, accelerating clouds effectively.

Shear in boundary layers causes slow cloud destruction.

Radiative cooling has little effect on cloud acceleration.

Abstract

We carry out a suite of simulations of the evolution of cosmic-ray (CR) driven, radiatively-cooled cold clouds embedded in hot material, as found in galactic outflows. In such interactions, CRs stream towards the cloud at the Alfv\'en speed, which decreases dramatically at the cloud boundary, leading to a bottleneck in which pressure builds up in front of the cloud. At the same time, CRs stream along the sides of the cloud, forming a boundary layer where large filaments develop. Shear in this boundary layer is the primary mode of cloud destruction, which is relatively slow in all cases, but slowest in the cases with the lowest Alfv\'en speeds. Thus the CR ray pressure in the bottleneck region has sufficient time to accelerate the cold clouds efficiently. Furthermore, radiative cooling has relatively little impact on these interactions. Our simulations are two-dimensional and limited by a simplified treatment of cosmic-ray dynamics, the neglect of CR heating, and an idealized magnetic field geometry. Nevertheless, our results suggest that cosmic rays, when acting as the primary source of momentum input, are capable of accelerating clouds to velocities comparable to those observed in galaxy outflows.