Simulating the X‐Ray Forest

Numerical simulations predict that a large number of baryons reside inintergalactic space at temperatures between 10^5-10^7 K. Highly-ionized metals,such as O VII and O VIII, are good tracers of this ``warm-hot intergalacticmedium'', or WHIM. For collisionally-ionized gas, the ionization fraction ofeach ion peaks at some particular temperature (``peak temperatures''), sodifferent ions can therefore trace the IGM at different temperatures. Weperformed a hydrodynamic simulation to study the metal distributions in theIGM. We then draw random lines-of-sight across the simulated region andsynthesize resonance absorption line spectra in a similar way to simulating theLy-alpha forest. By studying the distribution functions of H- and He-like O, Siand Fe in a collisionally-ionized IGM and comparing with semi-analytic resultsbased on the Press-Schechter formalism, we find: (1) ions with higher peaktemperatures (for instance, Fe XXVI) tend to concentrate around virializedhalos, which can be well described by the Press-Schechter distribution, ionswith lower peak temperatures are found both in small halos (such as groups ofgalaxies) and in filaments; (2) lower peak temperature ions are more abundantand should be easily observed; (3) peculiar velocities contribute a significantpart to the broadening of the resonant absorption lines.