THE NATURE OF THE WARM/HOT INTERGALACTIC MEDIUM. I. NUMERICAL METHODS, CONVERGENCE, AND O VI ABSORPTION

We perform a series of cosmological simulations using Enzo, an Eulerian adaptive-mesh refinement, N-body + hydrodynamical code, applied to study the warm/hot intergalactic medium (WHIM). The WHIM may be an important component of the baryons missing observationally at low redshift. We investigate the dependence of the global star formation rate and mass fraction in various baryonic phases on spatial resolution and methods of incorporating stellar feedback. Although both resolution and feedback significantly affect the total mass in the WHIM, all of our simulations find that the WHIM fraction peaks at z similar to 0.5, declining to 35%-40% at z = 0. We construct samples of synthetic O (VI) absorption lines from our highest-resolution simulations, using several models of oxygen ionization balance. Models that include both collisional ionization and photoionization provide excellent fits to the observed number density of absorbers per unit redshift over the full range of column densities (10(13) cm(-2) less than or similar to N-O (VI) less than or similar to 10(15) cm(-2)). Models that include only collisional ionization provide better fits for high column density absorbers (N-O (VI) less than or similar to 10(14) cm(-2)). The distribution of O (VI) in density and temperature exhibits two populations: one at T similar to 10(5.5) K (collisionally ionized, 55% of total O (VI)) and one at T similar to 10(4.5) K (photoionized, 37%) with the remainder located in dense gas near galaxies. While not a perfect tracer of hot gas, O (VI) provides an important tool for a WHIM baryon census.