  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.
