Cosmological Hydrodynamics with Multi-Species Chemistry and Nonequilibrium Ionization and Cooling

We have developed a method of solving for multi-species chemical reaction flows in non--equilibrium and self--consistently with the hydrodynamic equations in an expanding FLRW universe. The method is based on a backward differencing scheme for the required stability when solving stiff sets of equations and is designed to be efficient for three-dimensional calculations without sacrificing accuracy. In all, 28 kinetic reactions are solved including both collisional and radiative processes for the following nine separate species: H, H+, He, He+, He++, H-, H2+, H2, and e-. The method identifies those reactions (involving H- and H2+) ocurring on the shortest time scales, decoupling them from the rest of the network and imposing equilibrium concentrations to good accuracy over typical cosmological dynamical times. Several tests of our code are presented, including radiative shock waves, cosmological sheets, conservation constraints, and fully three-dimensional simulations of CDM cosmological evolutions in which we compare our method to results obtained when the packaged routine LSODAR is substituted for our algorithms.