Probing Cosmology with the X-ray Forest
  There is a growing consensus that in the present universe most baryons reside in galaxy clusters and groups in the form of highly ionized gas at temperatures of 10^6 ~ 10^8 K. The H-like and He-like ions of the heavy elements can produce absorption features - the so-called ``X-ray Forest'' - in the X-ray spectrum of a background quasar. We investigate the distribution of the X-ray absorption lines produced by this gas under three different cosmological models: the standard CDM with Omega_0=1, a flat model with Omega_0=0.3 and an open model with with Omega_0=0.3. We give a semi- analytic calculation of the X-ray forest distribution based on the Press- Schechter formalism, following Perna & Loeb (1998). We choose three ions (O VIII, Si XIV and Fe XXV) and calculate the distribution functions, the number of absorbers along the LOS to a distant quasar vs. redshift and column density in a given ion. We find that significant differences in the evolution of the distribution functions among the three cosmological models. Using Monte Carlo simulations, we simulate the distribution of X-ray absorption lines for 10,000 random LOS. We find there are at least several O VIII lines with column density higher than 10^16 cm^-2. Finally we explore the possibility of detecting the X-ray forest with current and upcoming X-ray missions and we present an XMM RGS simulation of a representative quasar X-ray spectrum. 
T. Fang and C. R. Canizares