Cosmic ray driven outflows in global galaxy disc 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 (CRs) 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 CRs. We find that including CRs naturally leads to robust, massive, bipolar outflows from our 1012 M⊙ halo, with a mass loading factor |$\dot{M}/{\rm SFR} = 0.3$| 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 CRs into star-forming regions can provide significant pressure support for the interstellar medium (ISM), suppressing star formation and thickening the disc. We find that CR diffusion plays a central role in driving superwinds, rapidly transferring long-lived CRs from the highest density regions of the disc to the ISM at large, where their pressure gradient can smoothly accelerate the gas out of the disc.