  We present a new N-body and gas dynamics code, called Nyx, for large-scale
cosmological simulations. Nyx follows the temporal evolution of a system of
discrete dark matter particles gravitationally coupled to an inviscid ideal
fluid in an expanding universe. The gas is advanced in an Eulerian framework
with block-structured adaptive mesh refinement (AMR); a particle-mesh (PM)
scheme using the same grid hierarchy is used to solve for self-gravity and
advance the particles. Computational results demonstrating the validation of
Nyx on standard cosmological test problems, and the scaling behavior of Nyx to
50,000 cores, are presented.
