Simulating the X-ray forest

Numerical simulations predict that a large number of baryons reside in intergalactic space at temperatures between 10(5) and 10(7) K. Highly ionized metals, such as O VII and O VIII, are good tracers of this "warm-hot intergalactic medium," or WHIM. For collisionally ionized gas, the ionization fraction of each ion peaks at some particular temperature ("peak temperatures"), so different ions can therefore trace the intergalactic medium (IGM) at different temperatures. We performed a hydrodynamic simulation to study the metal distributions in the IGM. We then draw random lines of sight across the simulated region and synthesize resonance absorption line spectra in a similar way to simulating the Lyalpha forest. By studying the distribution functions of H- and He-like O, Si, and Fe in a collisionally ionized IGM and comparing with semianalytic results based on the Press-Schechter formalism, we find the following: (1) ions with higher peak temperatures (e.g., Fe XXVI) tend to concentrate around virialized halos, which can be well described by the Press-Schechter distribution, and ions with lower peak temperatures are found both in small halos (such as groups of galaxies) and in filaments; (2) lower peak temperature ions are more abundant and should be easily observed; and (3) peculiar velocities contribute a significant part to the broadening of the resonant absorption lines.