Cosmic-ray Driven Outflows in Global Galaxy Disk Models

TL;DR

This study uses advanced 3D simulations to demonstrate that cosmic rays can drive massive, bipolar galactic outflows, significantly impacting galaxy evolution by regulating star formation and enabling gas escape.

Contribution

First 3D adaptive mesh refinement simulations incorporating cosmic ray physics to model galaxy outflows, revealing cosmic rays' critical role in wind acceleration and galaxy feedback.

Findings

Cosmic rays induce massive bipolar outflows with high mass-loading factors.

CR diffusion transfers energy from dense regions to drive winds.

Winds are multiphase and exceed escape velocities.

Abstract

Galactic-scale winds are a generic feature of massive galaxies with high star formation rates across a broad range of redshifts. Despite their importance, a detailed physical understanding of what drives these mass-loaded global flows has remained elusive. In this paper, we explore the dynamical impact of cosmic rays by performing the first three-dimensional, adaptive mesh refinement simulations of an isolated starbursting galaxy that includes a basic model for the production, dynamics and diffusion of galactic cosmic rays. We find that including cosmic rays naturally leads to robust, massive, bipolar outflows from our 10^12 Msun halo, with a mass-loading factor Mout/SFR = 0.3 for our fiducial run. Other reasonable parameter choices led to mass-loading factors above unity. The wind is multiphase and is accelerated to velocities well in excess of the escape velocity. We employ a two-fluid model for the thermal gas and relativistic CR plasma and model a range of physics relevant to galaxy formation, including radiative cooling, shocks, self-gravity, star formation, supernovae feedback into both the thermal and CR gas, and isotropic CR diffusion. Injecting cosmic rays into star-forming regions can provide significant pressure support for the interstellar medium, suppressing star formation and thickening the disk. We find that CR diffusion plays a central role in driving superwinds, rapidly transferring long-lived CRs from the highest density regions of the disk to the ISM at large, where their pressure gradient can smoothly accelerate the gas out of the disk.