AST 98-02568 David H. Weinberg Dr. Weinberg proposes to work with Dr. Katz (University of Massachusetts) and Dr. Hernquist (University of California, Santa Cruz) to conduct numerical simulations that model the cosmic history of the baryon component of the universe from z ~ 8 to the z=0, with particular attention to the Ly a forest, high redshift galaxies and the distributions of galaxies and intergalactic gas in the local universe. They plan to use TreeSPH - a numerical simulation code that combines a hierarchical tree N-body algorithm for gravitational components with smoothed particle hydrodynamics (SPH) for the gas component. The code now exists in parallel form. According to the leading cosmological theories, the galaxies and large scale structure that we observe today grew by gravitational instability from small-amplitude fluctuations in the early universe. Gravitational clustering of collisionless dark matter drives the evolution of the mass distribution on large scales, but astronomical observations reply almost exclusively on electromagnetic radiation that has been emitted (or in some cases absorbed) by stars or gas. Hydrodynamic simulations, which incorporate the non-gravitational physics that influences the evolution of the cosmic baryon distribution, dramatically enhance the predictive and explanatory power of cosmological theories. Preliminary results from the simulations show that a substantial population of high redshift galaxies is predicted by several variants of the cold dark matter scenario. The team is studying the clustering of Lyman break galaxies in different scenarios. Hydrodynamic simulations show that the low column density Ly a forest arises primarily in diffuse, smoothly fluctuating, intergalactic gas. The physics of this medium is remarkably simple, allowing the simulations to make robust, quantitative predictions given a specified cosmological model. Simulations of high redshift structure, supported by QSO absorption data, indicate that most of t he high redshift baryons reside in the diffuse medium that produces the Ly a forest, with smaller fractions in galaxies and hot gas halos. ***
