Cosmological Radiation Hydrodynamics with Enzo

We describe an extension of the cosmological hydrodynamics code Enzo to include the self-consistent transport of ionizing radiation modeled in the flux-limited diffusion approximation. A novel feature of our algorithm is a coupled implicit solution of radiation transport, ionization kinetics, and gas photoheating, making the timestepping for this portion of the calculation resolution independent. The implicit system is coupled to the explicit cosmological hydrodynamics through operator splitting and solved with scalable multigrid methods. We summarize the numerical method, present a verification test on cosmological Strömgren spheres, and then apply it to the problem of cosmological hydrogen reionization.