Gregory Bryan

The Santa Barbara Cluster Comparison Project: A Comparison of Cosmological Hydrodynamics Solutions

We have simulated the formation of an X-ray cluster in a cold dark matter universe using 12 different codes. The codes span the range of numerical techniques and implementations currently in use, including smoothed particle hydrodynamics (SPH) and grid methods with fixed, deformable, or multilevel meshes. The goal of this comparison is to assess the reliability of cosmological gasdynamical simulations of clusters in the simplest astrophysically relevant case, that in which the gas is assumed to be nonradiative. We compare images of the cluster at different epochs, global properties such as mass, temperature and X-ray luminosity, and radial profiles of various dynamical and thermodynamical quantities. On the whole, the agreement among the various simulations is gratifying, although a number of discrepancies exist. Agreement is best for properties of the dark matter and worst for the total X-ray luminosity. Even in this case, simulations that adequately resolve the core radius of the gas distribution predict total X-ray luminosities that agree to within a factor of 2. Other quantities are reproduced to much higher accuracy. For example, the temperature and gas mass fraction within the virial radius agree to within about 10%, and the ratio of specific dark matter kinetic to gas thermal energies agree to within about 5%. Various factors, including differences in the internal timing of the simulations, contribute to the spread in calculated cluster properties. Based on the overall consistency of results, we discuss a number of general properties of the cluster we have modeled.

Hydrodynamical simulations of the lyman alpha forest: model comparisons

We investigate the properties of the Lyα forest as predicted by numerical simulations for a range of currently viable cosmological models. This is done in order to understand the dependencies of the forest on cosmological parameters. Focusing on the redshift range from 2 to 4, we show that (1) most of the evolution in the distributions of optical depth, flux, and column density can be understood by simple scaling relations; (2) the shape of optical depth distribution is a sensitive probe of the amplitude of density fluctuations on scales of a few hundred kpc; and (3) the mean of the b distribution (a measure of the width of the absorption lines) is also very sensitive to fluctuations on these scales and decreases as they increase. We perform a preliminary comparison to observations, where available. A number of other properties are also examined, including the evolution in the number of lines, the two-point flux distribution, and the He II opacity.