Cosmic ray driven outflows in global galaxy disc models

Abstract

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