Galactic winds driven by isotropic and anisotropic cosmic ray diffusion in disk galaxies

TL;DR

This study uses high-resolution galaxy simulations to compare isotropic and anisotropic cosmic ray diffusion, revealing significant differences in wind formation, cosmic ray distribution, and magnetic field evolution, emphasizing the importance of anisotropic diffusion modeling.

Contribution

The paper demonstrates that anisotropic cosmic ray diffusion significantly influences galactic wind development and magnetic field amplification, highlighting its necessity in realistic galaxy simulations.

Findings

Both diffusion modes produce bipolar outflows.

Anisotropic diffusion delays wind development compared to isotropic.

Isotropic diffusion causes rapid cosmic ray escape from the disk.

Abstract

The physics of cosmic rays (CR) is a promising candidate for explaining the driving of galactic winds and outflows. Recent galaxy formation simulations have demonstrated the need for active CR transport either in the form of diffusion or streaming to successfully launch winds in galaxies. However, due to computational limitations, most previous simulations have modeled CR transport isotropically. Here, we discuss high resolution simulations of isolated disk galaxies in a $10^{11}\rm{M_\odot}$ halo with the moving mesh code {\sc Arepo} that include injection of CRs from supernovae, advective transport, CR cooling, and CR transport through isotropic or anisotropic diffusion. We show that either mode of diffusion leads to the formation of strong bipolar outflows. However, they develop significantly later in the simulation with anisotropic diffusion compared to the simulation with isotropic diffusion. Moreover, we find that isotropic diffusion allows most of the CRs to quickly diffuse out of the disk, while in the simulation with anisotropic diffusion, most CRs remain in the disk once the magnetic field becomes dominated by its azimuthal component, which occurs after $\sim 300\,{\rm Myrs}$. This has important consequences for the gas dynamics in the disk. In particular, we show that isotropic diffusion strongly suppresses the amplification of the magnetic field in the disk compared to anisotropic or no diffusion models. We therefore conclude that reliable simulations which include CR transport inevitably need to account for anisotropic diffusion.