The Nature of the Warm/Hot Intergalactic Medium I. Numerical Methods,
  Convergence, and OVI 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. 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 ~ 0.5, declining
to 35-40% at z = 0. We construct samples of synthetic OVI 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 <= N_OVI <= 10^15
cm^-2). Models that include only collisional ionization provide better fits for
high column density absorbers (N_OVI > 10^14 cm^-2). The distribution of OVI in
density and temperature exhibits two populations: one at T ~ 10^5.5 K
(collisionally ionized, 55% of total OVI) and one at T ~ 10^4.5 K
(photoionized, 37%) with the remainder located in dense gas near galaxies.
While not a perfect tracer of hot gas, OVI provides an important tool for a
WHIM baryon census.
