Cosmic-ray Driven Outflows in Global Galaxy Disk Models

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.